I'm using rftools-1.8.9-4.20beta11.jar in SP and MP server.

The slots in the storage block are greyed out and won't accept any of the tier or filter modules.

Hmm, this works for me. Can you show a screenshot?

That looks ok: Did you actually try to put a storage module in the top slot? What was that module called?

I tried tier 1, 2, 3 and they won't "take" in any slot.

That.s really strange. I cannot reproduce this

So, one of my players discovered that you can shift-click the modules into their slots when the GUI is open, but you cannot drag-n-drop them into the slots.

Is that the way they are supposed to work?

Drag and drop should work too. I'll investigate

Ok, I can reproduce this now. Will fix soon

Fixed in next release