Welcome to Discussions, MortimerSays
(1) First, check your iSight using Apple's
http://support.apple.com/kb/HT2090
If iSight works in every user account you try with all apps except YMM, your problem is NOT with iSight or SL. One of the following will be your best bet unless you get the help you need here.
(2) This YMM Help page says that the latest 3.0 BETA works with your
Mac OS X (10.6.1):
http://messenger.yahoo.com/
If you are not already using that, follow Yahoo's instructions for updating to the current version.
(3) If you still have problems and cannot find the solution you need via Yahoo's Help site,
http://help.yahoo.com/l/us/yahoo/messenger/
search or post your Yahoo specific topics in the appropriate Yahoo-dedicated user forum:
http://help.yahoo.com/l/us/yahoo/messenger/answers/answers-01.html
The YMM users who post there may have already found the solution to your problem.
EZ Jim
Mac Pro Quad Core (Early 2009) 2.93Ghz w/Mac OS X (10.6.1) MacBook Pro (13 inch, Mid 2009) 2.26GHz (10.6.1)
G5DP1.8GHz (10.5.8) G4 PowerBook 1.67GHz (10.4.11) iBookSE 366MHz (10.3.9) LED Cinema Display External iSight