Quoted from:
http://social.msdn.microsoft.com/Forums/en-US/commserver2009/thread/7729cd29-1921-4d13-89d1-d1a23fcf0684
To retrieve metadata for the UserProfile entity, use the following syntax:
var query = new CommerceQuery<CommerceEntityDefinition, CommerceModelSearch<CommerceEntity>>();
query.SearchCriteria.Model.ModelName = "UserProfile";
If you have strongly-typed wrapper classes, including a UserProfile wrapper class then this
syntax should be used:
var query = new CommerceQuery<CommerceEntityDefinition, CommerceModelSearch<UserProfile>>();