How do i use unloadMovie?

i have 2 blank movieclips in my scene, each one is there to load seperate external swf’s into when hitting a corresponding button. i need to write a script that will tell the first swf opened to unload when you load the second one. here is the script i have so far:

on (release) {
loadMovie(“email.swf”, “external”);
}
(that script is on the first button, external is the name of the black movie clip in the scene to load the swf into)

on (release) {
loadMovie(“thumbss.swf”, “external2”);

}
(that script is on the second button, external2 is the second blank movie clip.

i tried just adding a simple command like this to each button to unload one when the second clip is loaded, but it failed:

on (release) {
loadMovie(“email.swf”, “external”);
unloadMovie(“thumbss.swf”);

}
what do i do? any ideas? thanks!