You can use OptimizePress on WordPress MU / Multi-site setups with a few caveats:
1. You must be the owner of the multisite network – if you are not the owner you will have issues if you come to use some of our features which require admin control over the overall network
2. You will require a separate license/API key for each site that you create within the multi-site network
3. You should still only be building sites for yourself within the multi-site. Anyone found creating sites on a multi-site network for customer or client use will have licenses voided and sites removed from our system.