I've been trying to work around this issue for the last few hours. In my
document I have a bitmap background, and a few other bitmaps I exported from
Photoshop. Whenever I try to assign actions to buttons, it says the selection
cannot have actions assigned. However, when I make a new document, and make a
button from native tools in Flash (text, create rectangle), actions can be
assigned.

Has anyone else had this problem, and know how to fix it?

yep, yep for certain kg, however jagreff, the best place for you code is on the
timeline anyway. Just target the symbol instance name and add your code there,
usually creating a separate 'actions' layer is preferable to organize your code
in one location. things are a little different from the timeline, for instance
to use a buttons on handler use code like:

my_btn.onPress = function() {
gotoAndStop('_down');
}