Not sure if I'm being a total idiot here - I've purchased one of the packs on Steam and linked my account however can't seem to figure out how to actually enable it (get the membership etc.)
Is there a code? Does it just take time to appear on the account?
The only other explanation I can think of is that you might have accidentally created a new Runescape account and linked that instead of your current account.
If this is the case there is a link on that page for
"Change which RuneScape account is linked to Steam"