After registration on http://analyticsolver.com
You can download the application to use within Excel with these steps.
Opening Excel then a blank worksheet, go to the Solver Home tab and 'Log in' with what you registered on http://analyticsolver.com
From there you'll have access to the 15-day trial with the days remaining on this welcome/status screen. Or if you purchased then your license should be tied in and you'll see your purchased version along with the day left count.
If the login fails, you are most likely behind a Proxy Server. If this is the case some extra steps will need to be done to allow passthrough to access your license.