I had a similar issue with manage_notifications and tried to add it to the permissions that ADDPERM would grant, but ultimately it didn't stick. I suspect you may be in the same boat with SHOWPERM. I also couldn't use NOTICES. I suspect the code is a wee bit dated at this point and FB has changed their API's. Not sure -- haven't had a chance to do a deep-dive into it.
I hope this project doesn't sink. This is an awesome utility.