On Damus:
1) Click on your profile image
2) Open Settings
3) Go to Keys
4) Copy the Secret Account Login Key

Open Primal, if you have an account already setup
1) Click on your profile image
2) Hit the circle with the three dots to the right of your profile picture
3) Click in "Add an existing account"
4) Paste the Secret Account Login Key (should start with nsec)