I didn’t write the plugin with multisite use in mind. I’ll see if I can figure out where things are going wrong though. It probably has to do with the plugin page location being different in a multisite install compared to a single site install. Can you tell me the address to the Retail Menu page? For instance, on a single site its:
While looking into this problem I found that the form action I was using in the Add New Menu form was slightly incorrect. I updated it and just submitted version 1.0.1 to the WordPress repository. I can’t say for sure if this will fix the problem you are having in the multisite installation, but I think it might, Please let me know if you try it and it works (or doesn’t work).
Thanks for the feedback! And I’m glad people are finding a use for this!