Problem installing Alchemy Advanced with OMM - not recognising OBSE

[gnewna]

I've downloaded Alchemy Advanced from the Nexus, converted it to OMOD with the OMM, it shows up in OMM as green (can be activated, I think?) but when I try to activate it, it says I need OBSE v 0018 or higher, well, I've got OBSE v 0021 (downloaded within the last week from obse.silverlock.org) and as far as I can tell, it's installed correctly (I've also got DarNifiedUI and Better Cities activated through OMM and I think both of them rely on OBSE).

Am I missing something really obvious? I've installed OBSE to the Oblivion folder (the one that has sub folders including Data, and so on) as per the readme.