3Dsellers does not use MWS tokens, so there is nothing to generate or send; older instructions that mention MWS tokens or a Seller ID no longer apply.
To connect an Amazon account, go to Settings, then Integrations, then Amazon Accounts, click Add Amazon Account and sign in to Amazon (Connect Amazon Account to 3Dsellers (Seller Central)).
If your Amazon connection shows Invalid, click Renew Token and sign in to Amazon again (Reconnect a Selling Channel: Renew Token, Resync, Remove).