Are you trying to grant access so that users can use this custom entity but no other data at all?
You will still have to include access to all sorts of bits of CRM just to make the user interface work - especially the things on the Business Management and Customization tabs of your security role. You also need to check these 6 settings:
Special privileges in CRM Security Roles
If this is the only security role you plan to give to your users, I would suggest you start from a standard role and remove access to other entities, rather than start from blank and work upwards.
Hope this helps.
Adam Vero, Microsoft Certified Trainer | Microsoft Community Contributor 2011
UK CRM Guru Blog