Hi Divya,
Keep a break point in Get Method of your attribute and check whether the selected value in F4 help is coming to VALUE field in Get method. If you create custom genil entity you have to create few custom class like BTIL.
Please refer to the below link
Extend BOL Model BT with custom table type relationship - CRM - SCN Wiki
I think you can use AET option to create table for existing BTOpportH entity.So that all the standard classes will be generated automatically.