Being part of the Express Administrators group should suffice, but you can try if making the Express Administrator account a member of the Express Users group solves your issue.
Can you check the permissions for the EVStudio capability?
- launch IBM Cognos Administration from Cognos Express Manager
- on the Security tab click on Capabilities
- click on the little down error near EVStudio and select Set properties
- on the Permissions tab grant Execute permission to Express Administrators