first off, a macosx icon isn't just a single 128x128 bitmap. it can be
up to four sizes, ranging from 128x128 to 16x16. you can skip one or
two of the middle sizes, but you really better not skip the 16x16 one.
if you do, macosx would have to create one of that size when it needs
to, by scaling your 128x128 icon, which will probably look like crap.
you don't say what tools you used to create the app. when asking
questions like this in the future, you should include that information.
i'll have to assume xcode. it comes with an app called "Icon Composer"
you can use to turn your bitmaps into an .icns file. you should drag
that .icns file into the resource section of your xcode project.
there's some other fields in the project you have to fill out to tell it
to use that .icns file as the app's icon. i can't remember exactly what
they are at the moment, but if you poke around in there awhile, it's not
hard to find.