I try to install AppBrain, but I keep getting "page not found" or "error 404" when I click the link in the welcome email.

You need to make sure that the device you're trying to install AppBrain on is an Android device with the Google Android market installed on it. The Google Android market icon looks as shown on the right:

If you do not have this icon on your phone, you can not use AppBrain, as all the apps are downloaded through the Google market.

Getting a 404 or "page not found" may be caused by using a special browser on your Android device. If you don't want to switch browsers, you can also get the AppBrain app through the Google market: go into the Google market (by clicking the icon as shown on the right) and do a search for "AppBrain" and install from there.