First: Go to google and type in Too Many Items (should be the first one as minecraft forums)

Second: Download it.

Third: If .zip file then double-click it and make it a folder on the desktop

Fourth: Open the folder for TMI and the open a new finder window and go to library>application support>minecraft>bin then right click (control click) on minecraft.jar and open with archive utility then delete minecraft.jar (it wont delete minecraft TRUST ME!) then rename the folder to minecraft.jar.

Fifth:go into minecraft.jar and delete METAINF then drag on drop all the items from the too many items folder into minecraft.jar folder then test it and it should work

If it doesnt tell me in comments

Moar tutorials for mods at 5 diamonds!