Hi sira1234,
Welcome to Auctiva! In order to be able to select a seller details profile, you must first select a template. Therefore, you would only be able to build your seller details profile into your item details section of your master if you are selecting a template within your item details profile.
If you choose a template while you are creating your items details profile, the "Select Seller Details" drop-down menu to the right should no longer be greyed out and you should then be able to select any of the seller details profiles that are saved within your account.
If you won't be using the same template for all the listings you'll be creating from your profile, you could always omit the selection of the template and seller details profile from your item details profile and select those things manually as you create each individual listing.
If you have any additional questions regarding your Auctiva account, please feel free to contact our customer support team using the web form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx
-Mike