As you have found Gimp has a learning curve. There are Gimp forums around which will answer all your questions, especially if you have problems with a tutorial.
Although your post was about Gimp pattern files, you can use a regular .png file as a pattern. Might/might-not be tile-able, that is another subject. These are examples of png files from my patterns folder.http://i.imgur.com/ELNhG.jpg
Why so few? - not showing the default patterns?
If & when you get into using Gimp, there are dozens of plugins and scripts, one is GURM - a gimp resource manager which lets you swap out brushes, patterns, gradients etc. I have any number of patterns, but none anywhere near the Gimp global folder, I swap them in & out as required. see:http://registry.gimp.org/node/13473
As with anything it is about use and gradual learning. But Gimp is a great application, been using it for years, do not be put off.
the tick box in the preferences -> patterns only makes the folder(s) writable. Any suitable file in any of the folders listed will show in the toolbox.