When you start ET the first thing you see is the MAIN menu with a list of options. If you have correctly placed the Fritzbot folder in the Enemy Territory folder, MODS will show up near the bottom of the list just above EXIT.
Select MODS and then choose Fritzbot and it'll load. Then click START, choose a map to play, then click START SERVER. If it is a currently-supported map it should start up with a selection of bots to which you can add more with the /addbot command.
You can eliminate the MODS procedure by making a shortcut with the command line "Enemy Territory\ET.exe + set fs_game fritzbot" and it'll start you up in Fritzbot right away.
If this doesn't help, please give us some more detailed information about exactly what steps you have gone through and what happens.