carbon is selected and the lazarus directory is filled in correctly ; (
Not sure what to suggest.
Are you using today's snapshot? Maybe something got broken. That happens quite often with the snapshots since they're not stable builds.
Check to see if the carbon widgetset has even been compiled. Normally this is included with the snapshot. Look in your lazarus folder under lcl/units/i386-darwin/carbon (or lcl/units/powerpc-darwin/carbon with older Macs). If the interfaces.o/.ppu files don't exist, you'll have to compile the carbon widgetset. (Tools | Configure. Set everything to None except LCL, then click Build.)
Also, check to make sure you're really pointing to the correct lazarus location. If you moved it, this might have changed.
Thanks.
-Phil