Temperature Monitor was last updated in 2009. So maybe it is not compatible with your OS.
I recently abandoned the fantastic MenuMeters (free) for iStat Menus 5, since the later provides even more info on practically all Mac sensors. Hardware Monitor and System Monitor are also capable of showing sensor information in the menu bar.
I know that this is an old question, but by the end of 2020 there is a really nice open source utility — iGlance. Just tried it and it seems to work really fine. There are some minor issues with appearance under some circumstances, but otherwise it is very solid.