Why can't I make this usb drive bootable?

I’m using usb creator, trying to make a bootable pen drive for an eeepc.
When I open the program and browse for the iso file, the option “make startup disc” isn’t even clickable
(as a matter of fact one of the two iso files i’ve downloaded doesn’t even appear in the top list when I select it….. but the other does..)

Please help, quite an emergency.
Note: I am an amateur in this field, but I’m trying my best, so…. go easy on me ;)