The StatET package for Eclipse works great on a Mac. Go to StatET and follow his installation instructions. Sometimes it can be a little tricky to set up once the package is installed in Eclipse. There are several cheat sheets in Eclipse to assist with the setup. Look for them under Help->Cheat Sheets in Eclipse.
If you are using Eclipse 3.7 Indigo, you can drag and drop the plugin from the Eclipse marketplace.
Edit: There are many possible R development environments that will work on a Mac. Look at this question. Most of those run great on a Mac.
solved Eclipse and R on Mac [closed]