From the Library view select Edit.
Tap 'Add Location'.
Select Working Copy repo and hit Done.
Hit Done on the Library to finish.
I had to scrabble about a bit to find out how to open Working Copy repositories from iA Writer (on iOS), so I thought I’d document it here.
Basically you need to add it to your Locations.
Bring up the Library view in the left hand pane of iA Writer and hit the Edit button.
Tap the Add Location… link.
Then, in the panel that comes up, you should see a Working Copy entry. Expand that and select the repository or directory you want to add. Then hit the Done button at the top-right of that panel.
To finish, hit the Done button at the top-right of the Library panel.
The Working Copy directory will then be added to your Library under Locations.
You can now add and edit files in that repository as you choose and then, when you’re finished, just go into Working Copy and commit and push the changes.