Let me say first of all that I'm a rank beginner with RB.

A friend sent me a mp3 file of him playing a guitar. I loaded it into RB and then recorded a couple of fiddle tracks to go along with it.

My friend then sent me a second mp3 containing a harmonica part. I can import it into RB, but can't - for the life of me - figure out how to slide it back and forth on the track in order to align it with the other tracks.

First, there must be room at the front bars of the track to allow lateral motion in either direction. So, if the audio is starting at the very beginning of the track, the track can only move to the right. Several ways to overcome this limitation and I think the easiest and way I use most often is to import the audio track a few bars into the song in anticipation of aligning with the other tracks.

With room to move taken care of

Ctrl-A to select the entire track
Hold down the control key and place your cursor onto the track and while holding the control key down, you can slide the track in either direction.

A tip to accurately align two tracks to each other is to place the cursor on the track you're moving to a point directly on the WAV form that you know the point on the track syncs to a point of the track you're syncing to and then use that reference to guide your slide movements.

Oh, thank you so much! That worked beautifully. I spent an hour or so searching the documentation, but apparently I just wasn't using the right search term.

Glad you got it sorted. It took me a while to figure out how to move a track to the left.

it can be tough to drag an audio track in very small increments. One way to make precise moves in either direction is to:

1) hilight the entire track (CTRL-A). A dialog box should appear with a starting and ending point for the highlighted area
2) take note of the starting point
3) paste the track (CTRL-V). The same dialog box with a starting point for the paste operation.
4) Edit the starting point by whatever precise value you need before hitting ENTER to complete the paste operation
5) if the value you entered doesn't slide the track the correct amount, CTRL-Z to undo the move, then try again with a new start point

note: if you need to slide the track to the left, when you copy the track you will need to edit the starting point for the copy operation so there is room to the left. You can't move to the left if you started copying at the beginning of the track. This is basically what Charlie already said, except he was describing how to drag the track while I am describing how to use the dialog box to input a new location

The accuracy of such slide adjustments is all in your resolution settings.
The higher your resolution the more you can zoom in and accurately slide things to where you want them.

If your Resolution is 120, you get to slide it to the nearest 120th of a beat over time .. but if your resolution is 3840, well you can be much more accurate with any timing related edits you do.

The first image below is given as an example.
With the default 120 resolution I could move the track to either the 114 or 115 locations.
With 3840 resolution I could move it much closer to the location I desired.

It's an old image, and I think in the original posting of it I was referring to how much more accurate a users 'cuts' could be, but the same applies to other edits, like sliding a track.
The higher your resolution the more accurate you can be .. just have to get used to using bigger numbers and how to zoom in for accuracy!
The second image shows how to adjust said resolution

If both DAWs are set to the same tempo no sliding is required. Ive done 2 Cds worth this way with a friend
.

IF they are smart enough to send the whole track! I had a guy do some harmonica stuff for me, and he snipped JUST the parts of the track where there was music and sent them to me as wav files. With notes like "Try fill 1 v3 in the first section" and "There are 2 solo sections, both with different entry points. The are called Solo1 and Solo2." Cue the nightmare sound effect here. OMG what a nightmare to time his syncopation and pickups with no point of reference. It ended up being great though, thanks to him being an AMAZING harp player and me being an AMAZING frickin' producer! LOL!!! I think he sent me like 35 small clips, though that made for it being very much like assembling RB solo snippets.

Yea I tell to send the whole track