Follow SMF on Twitter.
First, you should upgrade to SMF RC4 ASAP, there were several bugs and security fixes patched since RC2. This may also fix your problem. Please read Upgrading SMF.
Check to make sure that karma is enabled in the first place. If so, check the default theme to see if the karma buttons work there.
Is there anything in your error log?
Page created in 0.089 seconds with 17 queries.Page served
by: 10.0.100.135 (10.0.100.113)