Rival Ball will automatically search for installed board packs every time you launch the game, through pre-defined paths. These paths are written in a file named 'dirs.cfg', which is located in the main directory of Rival Ball. However, if the board pack you're missing wasn't installed in one of the following paths, Rival Ball won't recognize it:
C:\Program Files\Rival Ball\boards\
C:\Program Files\DX-Ball 2\boards\
C:\Program Files\LDA Games\DX-Ball 2\boards\
C:\Program Files\LDA Games\Rival Ball\boards\
I suggest you re-install the board pack, but this time, in the same directory where Rival Ball was installed.
Hope that helps.