Save an asset

After inserting an model asset into my model, how do i save it to my hard drive, when i click on it, the export button is greyed out ( all 4 of those buttons are greyed out ) ?
Also if i buy some extra assets what format does it need to be in
Thanks Jason

Greyed out? Could you provide with screenshots?

here Jessie

The models from D5 built-in library cannot export. These 4 buttons are only available for your own models.

OK thank you, if i buy some from elsewhere, what format should file do i need ?