Enum Guide
You can't add a drop-down property to custom node groups. This means that there's no direct way to expose a node input as an drop-down item to the property controls. There's a better fix: built-in enum node (Menu Switch), but somehow the menu selection can't be animated so it's pointless.
Beantextures uses a separate enum property that resides under instances of node groups. This property would then drive the Value input of the Beantextures node it's on. The drop-down that you get in the Beantextures controls UI is actually the enum property, not the actual node input like with the int/float linking types.
When you use the enum linking type, make sure to apply the needed driver by clicking the target node and then hitting "Add driver" under inspector (in Beantextures' panel):
