Thanks, but .KAR is different than .cdg
.KAR files are basically MIDI files with Lyrics in an Event.

.cdg files contain video lyrics

Here's how I open the .mp3 file that has an associated .cdg file in RB.

Start RB.

Select Open file (either menu or toolbar button)
Change to directory that contains the .mp3 and .cdg files.
Select the .mp3 file
Open it.
RB loads the .mp3 file to track 1 and opens the Karaoke Lyric window.

Note: The Karaoke Lyric window is different from the Big Lyric window that opens with SEQ files or KAR files that have lyrics.