The online functions for legacy Command & Conquer games are now managed by an organisation called XWIS, you can no longer use the "Account management" page to make an account. So how do you make an account? Simple, just do the following:
1. Install Renegade.
2. Click Multiplayer in the main menu.
3. Click Westwood Online.
4. Click Advanced Game Listings.
5. Type in ANY user name you want to use, as long as it isn't already taken, it's yours forever and ever.
6. Type in a password that is EXACTLY 8 characters long. No more, no less.
7. Click Login
8. Join a server.
If the sever listing is blank that means either that your password was incorrect (if you are a returning user) or the name you chose is already in use (if you are a new user).
If you have a router you may need to open ports, these are listed in this thread at Strike Team:
Frequently Asked Questions
99% of online problems are answered in this thread at Strike Team: