Eclipse used to need a column mode plugin to be able to select a rectangular selection.
Since Eclipse 3.5, you just need to type Alt+Shift+A: see its News and Noteworthy section. (On OS X it's Option-Command-A.)
Or activate the 'Editor Presentation' action set ( Window > Customize Perspective menu) to get a tool bar button for toggling the block selection mode.
AmbroseChapel adds in the comments:
This is a toggle.
Columnar selection is a mode you enter and leave: in other words, Eclipse switches into a mode where all mouse selections have to be columnar and you stay in that mode until you switch back (by using the same command again).
It's not like other editors where columnar selections are enabled only while certain keys are down.
