Hi Dean,
The best way to do this is to use the External Datasource (Tools | External Datasource). Set the Manufacturer and Model Number attributes in the symbol as normal and ensure there is a corresponding entry in the External Datasource spreadsheet. You need a tab that shows the manufacturer name, then add titles in row 1 for the attributes you want (the Title in Column A should be "Model Number"). Attribute values will be added when you drag the symbol into a document.
You can read up on the external datasource functionality at http://www.stardraw.com/products/stardrawav/NewFeature.asp?date=0310
I hope this is helpful - let me know if you need any other information.
Kind regards
Rob