Liking the game so far and it's been running without any problems. However, after getting the Asson weapon and the Engineer's Key, I can't figure out how to switch to them. I'm currently in the Asylum and just got the Engineer's key, but I'm trapped in the room since you need to equip it and use it in order to get out. I've been able to press switches using Mouse3 fine, but for some reason nothing works in the inventory menu... I really want to keep playing this game but it seems like I'm stuck.

The post was written more than a month ago and the OP has probably figured it out in the meantime, but here is the answer for anyone else who is stuck with the same problem:

The game uses a system that will be familiar to anyone who played the Dark Souls series. Namely, the protagonist has a left and a right hand, and can use a different weapon in each. Go to the keyboard config menu and find the keys bound to "left hand" and "right hand" respectively. Remap them if you wish. Those are the keys that, if pressed when an equippable item is highlighted in the inventory menu, will equip the item in the left/right hand. Then you can use the item using the left/right mouse button when controlling your character. This is very useful because you can have a combo of items equipped, say your wraithgun in your right hand and a voodoo item in your left hand to give you an extra edge in combat.