Alright, so after doing some research, it seems that Lightning Labs' LN address is not usable for zaps on Nostr (yet, hopefully). Instead, one needs to use a "lightning network-compatible wallet that supports zap" (email-address like), which can be offered by many other apps. I just set up mine via WalletOfSatoshi (on Android), updated my Nostr profile and should now be ready to be zapped on Nostr.

Not very user-friendly yet, but we are still in the early days. ⚡