The client does nothing — confirm you signed into the client with the SDN email, not only the forum login.
No scripts in the client — buy a subscription and wait until the account is marked paid.
Cannot post on the forum — Sign Up or Sign In. Guest posting is off. bbPress handles accounts.
PayPal subscriptions — pick a plan on your profile. PayPal opens on paypal.com; you come back here after you pay. Rank turns on as soon as PayPal confirms. Keep your account number (UB-…) and invoice (INV-…) from the receipt email.
Server donations — the gold Donate bar (UnrealBot logo + unrealbot.co.uk) goes to /donate. That is PayPal too, and it does not add subscription days.