File
The path and file to a testable file. You can specify a dll or a Visual Studio/NUnit project file.
...
Select the .NET framework to run the tests under.
Using
The Using drop down is populated with any property collector whose namespace matches the pattern defined by the NUnit Action. The pattern for this action is ^NUnit.*
If you create a property collector for this action, make sure you select the Path Finder PlugIn type and give it a name that will match the pattern above in blue. Example names listed here, search the table's Plugin column for "NUnit".
For more in-depth explanations on property collectors see Managing Property Collectors.
Alternatively, you can select the Custom option from the Using drop down list and specify a path in the resulting input field that will be displayed. Please read Why it's a good idea to use a property collector before using this option.