You should equip a weapon first ( it's the knife from your inventory, click on: Equip a weapon!, you should be directed to a screen with the weapon availables, click on the equip button under the weapon icon) and then choose embark on an expedition and you should be able to start your play.
If nothing happens the only thing I could think is maybe something went wrong when you installed the application ( or a possible bug, but I play from an Imac, and I never used the android app so i don't know how it work, sorry.