you have to make the pattern into a seamless tile. My favorite way is using this filter:
http://www.btinternet.com/~cateran/simple/#qt
To use it, save the plugin filter anywhere on your hard drive. In PSE go to Edit>Prefs>Scratch Disks and Plug-ins and at the top of the dialog set that folder to the one where you save your plug-ins. Reopen PSE and it'll be at the bottom of the Filter menu, called Simple.
Select one "tile" of your pattern and crop the image (Image>Crop) and run the filter. Edit>Define Pattern and it'll be at the bottom of your pattern menu. You'll have to save the pattern as a pat file with the Edit>Preset Manager>Patterns and select the pattern and click Save
hth! There's a tutorial for doing this manually witht he Offset filter at
www.myjanee.com too