Boolean operations disabled

Every time I try a boolean modifier, I keep getting a “modifier is disabled” message using Blender 2.75a on a mac. Can anyone tell me how to enable the boolean operations?

Numerous reasons. Please always supply a demo blend file with support questions. You can upload your blend file to http://www.pasteall.org/blend/ then tell us the download link

I solved the problem. The second object of the boolean operation needed to be specified as part of the modification which is different than my last doing a boolean operation. The “disabled” message was misleading. I think the message should be “no modification object selected” or something of a similar nature.