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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.