You should have been routed to the server after signing up. If you didn't, you can use the "Generate A New Invite" link found on your welcome email. Please use this only once and keep your account logged in to avoid future access issues.

After getting into the correct server, send the activation code to the Discord Bot on the right side of server in in the user/member list.

The bot looks like this:

These are the different areas on Discord. The user list should be on the right most panel: