Ctrl–/ to toggle “//” comments and Ctrl–Shift–/ to toggle “/* */” comments. At least for Java, anyway – other tooling may have different shortcuts.
Ctrl–\ will remove a block of either comment, but won’t add comments.
Note: As for Eclipse CDT 4.4.2, Ctrl–Shift–/ will not uncomment a “/* */” block comment. Use Ctrl–Shift–\ in that case.
EDIT: It’s Ctrl on a PC, but on a Mac the shortcuts may all be Cmd instead. I don’t have a Mac myself, so can’t easily check.