Hi There.

1. Open the Evermée App, Login and click on "My Collection"
2.  Click on the 3 dots behind the méemory (haha "mée") that you want to give extra photo slots 
3. Open the settings and type your code into the "additional memory code" field 

You can find your photo slot code inside the purchase email "Evermee: Your Extra Photo Slots"

Done :)

*1 code, even a 10 photo slot code can only be used for 1 memory.