You can automatically enable Simple:Press forums permissions to users who purchase particular subscriptions. To do this, you should first create your forums and security groups in the Simple:Press Forums plugin.
Then, in each membership level in the Simple:Press Member Subscriptions plugin, scroll down to the Simple:Press Forum User Group section and assign a user group to the membership.
Note: If you have enable the ROLES function in SPMS, then you should use the ROLES integration option under FORUM->USER GROUPS->MAP USERS TO USER GROUP instead.
Why would you use the roles method?
Sometimes, you need to use roles to integrate with other plugins that do not have direct support for our memberships. If you have to turn on roles support (which you can do under MEMBERSHIPS->SETTINGS->COMPONENTS), then you would use the roles integration option in Simple:Press Forums. Otherwise, it is easier to specify the integration directly in the SPMS membership levels screen.