Open Impulse, click the icon in the top left corner and select 'Register a product...'. In the bottom right corner select 'Add registration' and fill out the form. Impulse should then reconnect or restart and then show your purchase in the 'My Games' tab.

If that doesn't work (sometimes using different email adresses causes problems): write to [email protected], explain and they will help you.

You could also try to uncheck 'Show hidden applications' (which is only clickable if anything is hidden).

Hope that helps!