I'm running OS X 10.7.5, 2.5 GHz i5, 16GB Ram, Radeon HD 6750M. Your new version of GUI Miner does not detect my GPU with the error message "No OpenCL devices were found" however the older version of GUI Miner that was built for Mac 2012-11-18 version does detect my GPU and works fine for mining.
I got the same error on the same GPU (Radeon 6750M) and on Lion. I tried both the older (and otherwise working with the old GuiMiner) 2011 POCLBM and the latest one as described neither make any difference, the problem seems to be your version of GuiMiner.
Any chance of a fix?