If you're not sure what ISRC's are, start here. If your songs are not on Spotify, but have been distributed, you can obtain your ISRC's by contacting your distributor.
When you're logged into your Songtrust account, you can add recordings from your 'Songs Page'. Adding ISRC's from either searching Spotify or entering them manually helps you attach your recording information to your compositions. This is so you will be recognized and owed royalties whenever the recording is played.
On the 'Add Recording' page, if your songs are not on Spotify, click the 'Manual' tab to the right. Once you paste your ISRC , a 'plus sign' appears in the very right of the screen. Select 'Confirm ISRC is Valid' and continue to manually add your recording info i.e. Artist name, genre, etc.