I imported brokerage data. TT not asking who account belongs to for stock trades; only DVDs and INT. How do I tell TT who to assign those trades to?

why do you think it matters? on a joint return except possibly in a community property state whether it's yours or your spouse's doesn't matter.  

TurboTax will ask you about ownership of stock sales if and when necessary. 

Marital property is considered jointly owned under federal tax law so there is no need to break out ownership.

As Tax Champ Mike9241 says, ownership will not matter except if you are filing separately in a community property state or in some state situations. In those cases, such as Pennsylvania, there will be specific questions about ownership in the state section.