1. Download and install SimpleText for Mac
2. In Notational Velocity, choose Preferences > Notes > Read notes from folder:¹
3. Select the SimpleText folder in your home directory.
4. Click the Storage tab.
5. From Store and read notes on disk as:, select Plain Text Files.
6. That’s it! Notational Velocity will now use the SimpleText folder as its note repository, which is kept in sync with the SimpleText.ws service and by extension WriteRoom for iPhone.

¹ Alternatively, SimpleText can be made to use the existing Notational Velocity notes folder via the Terminal command:
defaults write com.hogbaysoftware.SimpleText SyncedDocumentsPathKey ~/Library/Application\ Support/Notational\ Data/