Enabing Modifyers.

Every time I try the “add modifier” like t do a Boolean" Difference I get Modifier disabled bypass apply-- message popup.

HOW do I ENABLE the modifier?

Have you selected another object to use for the difference boolean ?

Supply an example blend file that will show the error when we press the Apply button

I am trying to cut a cylindrical part out of the cube—You can see the error message at the top

Have you selected another object to use for the difference boolean ?

Supply an example blend file that will show the error when we press the Apply button

I selected the cube AND the cylinder–only the cub selection outline is no on my image when I snapped the screen snip.–You can see the error message though

Did you add the second object in the boolean modifier settings
Read the manual http://www.blender.org/manual/modifiers/generate/booleans.html

Please NEVER crop your screenshots unless you know why you don’t need to. You have cut off important info such as the boolean modifier settings. Also ALWAYS supply a link to your .blend file for ALL support questions. Otherwise all anyone who may want to help has to rely on is guesswork