I also have this problem as well as several other steam users: http://steamcommunity.com/app/427520/discussions/0/358415738202058287/
Please resolve this issue so the mod portal can be accessed.

We are having some teething issues with our steam linking showing memberships as free, please bear ...

By the looks of several other threads I have seen, this issue might not be an isolated one.
I have the same problem and additionally the multiplayer wont let me connect for the same reason.
Here are two other threads that seem to have the same problem, or at least a very similar one.
https://forums ...

I can confirm I am experiencing the same issue. Either attempting to login via the install mods button, or via the multiplayer I get an error about insufficient membership. I am using the correct account, at least I think I am as when I login using the same username and password on the website my ...

I only just now realised the date you posted this. I was so excited about the idea mobile factorio (even if the exact details of the implementation looked a bit strange). At least finding out gave me the incentive to actually create an account here