creating a watermark?

i have aperture2 and the borderFX plug in. i know how to go about adding the watermark to the photo, but i don't know how to make the actual watermark to add to the photo. i made one in the word processor in the Mac OS, but when i try to add it, all i am allowed to add, is a picture... could someone please help me

You need to create it as an image in something like Photoshop. It needs to have a transparent background and be saved in a format that supports transparency such as PNG. Word processors are for word processing, they can't help you here.