index: false will cause it to not show up in the acp page. Remove that.
warn: [plugins/nodebb-plugin-mentions] This plugin may not be compatible with your version of NodeBB. This may cause unintended behaviour or crashing.
This is the error on the log i get
Yesh... that warning means NodeBB reports itself as v0.4.3 (because v0.5.0 hasn't been released yet), but the mentions requires v0.5.0 (which the latest commits do satisfy)