You need to purchase the isoft plugin from amember. Just send them an email or post a support ticket. The plugin is $40 I believe.
In Infusion set up tags for every membership you'd like to sell. (for example Gold Member Active). Set up an action set or include the set tag when you sell the membership through the Infusion shopping cat or Order form.
The Isoft plugin of amember let's you see a list of all tags in Infusion in the product set up page. You select the tag you set up in Infusion and save.
Set up the cron job as explained in the amember documentation.