try using the duplicate tool...
Edit > Duplicate click on the options box.
you'll have rotate, scale and transform boxes in there, three of each, they are equivalent to x,y, and z respectively.

change your transform number in the correct axis for your situation , "What ever it may be" to either 1, or -1 to duplicate strait across the transform node for a perfect miror copy.
You'll use either a negative number, or a positive number depending on which side of the axis you chose to model your base on .

Personally i prefer this method anyway, it just seems to work cleaner , at-least to me... plus you can instance your geometry so that the duplicate automatically updates with the edits to the original.. similar to symmetry option in Zbrush and 3DS. far as i know mirror doesn't have that option. though i might have overlooked it, since i only used it ...2 times i think.

this duplicate function also has a lot of other uses. you want to duplicate something in a circle around the y axis for instance, you can take the number of copies you want to have, divide that by 360, change the rotate setting for he y axis to the number you derived form previously mentioned equation. just as one good use for that other then mirroring geometry.

g-man