I pass this information on [I do not use the program]:

right click in the thumbnail area at the bottom of the screen and click on 'preferences'

under excluded items there are two buttons you can click:
'file format exclusions' and 'uncataloged folders'

1) click 'file format exclusions' then 'select all' then click 'ok'

2) if the above does not help repeat, but click on 'uncatlogged folders' and click 'reset to default'

check also these two threads here:
http://www.talkgraphics.com/showthread.php?t=23575
http://www.talkgraphics.com/showthread.php?t=33796

this is general information - it may not apply to your version or system