1) make sure your user token is actually in your clipboard by trying to paste it somewhere else (e.g. notepad, your browser's address bar, etc) if you can't paste it anywhere, it means it wasn't copied to the clipboard. Try selecting and copying it manually, rather than using the Copy button next to it.
2) you can't right-click to paste in-game. Press CTRL+V on your keyboard to paste, make sure the text field you are trying to paste into is selected (should have a blinking cursor in it) and press ENTER to save.
3) make sure you are pasting it into the correct text field (Multiplayer>Settings>User token
4) make sure your player nickname
matches your forum name exactly - "Boss Hogg" with the capital letters, with the space, if it doesn't - it won't work.
It's possible that you made a mistake when typing it in by hand, that would explain why it isn't working, or your player nickname didn't match your forum name.
Also there is this painless workaround for "can't paste"
what should i do
Documents\My Games\Rigs of Rods\config\RoR.cfg