Have you connected your factorio.com account with your steam account on the profile page (click on your name on the top right)?
If it still doesn't work, then
Should all be fixed now, please recheck your accounts and email me (support[at]factorio.com) if you still show as free membership
as per ...

Hello, I purchased on Steam and I'm trying to download some mods from the mod portal here. When I log in to download one it says I have Insufficient membership. What should I do?