Please bookmatk this link to the FAQs so you can check there before posting to the forum. http://faq.wordpress.com/
As dctage has indicated, there are differences between a post and a static page. We cannot assign categories to static pages.
Please also consider linking your username to your blog. It’s very easy to do. Here’s how to link your username to your blog in your blog Dashboard –> Users –> Your Profile. Then scroll down to where it says ‘Primary Blog‘ and fill in the address of your blog and then click ‘Update Profile‘.
Once you’re done click “Update Profile”. Voila! Now every time you sign into the forum we can all click on your username and your blog will be linked to it. And every time you leave a comment on a blog your username can be likewise clicked and your blog can be instantly located.