Hi Ed.
So your drop down is working properly? You can use the Value and Display name at the bottom of the properties to set it to display one field while containing another.
Are you using a filter on your set field value action? For Field: you would lookup the field in your view which should contain the name. For Value: you would click on the function button (FX)
Then Insert Field or Group
Then use the drop down at the top to select your secondary data source
Select the account name field and then select filter in the lower left of the window.
Add a filter
Accountnum = your drop down field in your view
Ok everything
This will return the account name which the account number = what was selected.
Hope that helps