Regarding what Ed said in line 4 of his answer, you right click on the label for the instrument you are wanting a RT for (Piano, Bass, Guitar, Strings). One of the options that pops up in a menu is to Add/Remove Real Track. When you select that it will take you to the Real Tracks list. One option you might not want to have checked there is to Show Real Tracks that are N/A. If you don't check that, you will only be presented with a list of RT's that you have on your system. Pick from the list for whatever instrument you are looking for. You can do this for all instruments from this window (upper left area of the screen lets you choose the BIAB instrument location you want to change). Once you have the RTs you want, exit out and play along with your song. It is great fun.
Good luck.