I've got a series of photos that will walk you through it. Remember, if you have any questions at all, feel free to leave me a comment, or drop me an email. I like helping people! This tutorial is for Adobe Photoshop CS5, but I think it's similar enough to earlier versions that you can figure it out.
Once you've got your action saved, you can play it on any image, and your watermark text layer will appear. Then, you can drag it into place, resize it if you like, change the color or opacity... whatever you want. You can also set the color and size and things when you set up your action. Just make sure the watermark looks basically how you want it when you're recording it, and you can save yourself lots of time and headache in the future!
Hope this helps, and I hope you have a great day.