First you should not store the "Category Desc" in tblProduct, use the CategoryID instead. As far as the question of filling up the combo box...
Use a SQL query like this : SELECT NAME, CATEGORYID FROM TBLCATEGORY
use this to fetch the data and bind the combo box with the datatable.
set the data member and value member with the "Name" and "CategoryID" respectively.
This article contains a small sample, would be helpful:
http://msdn.microsoft.com/en-us/library/x8ybe6s2(v=vs.80).aspx[
^]