Macbook refuses to use 802.11n?

Question

I have an 802.11n router, running in MIMO mode with WPA2/AES encryption, with only two machines attempting to connect to it. Both are running OSX 10.6.5:
My macbook air 1.6GHz (December 2010). This machine connects perfectly as 802.11n, every time.
My wife's 2GHz Macbook 2,1 (I think somewhere around mid-2007 manufacture). This machine has the n extension installed (I can't recall installing it, but network utility shows it as 802.11 a/b/g/n). Whenever I attempt to connect with this machine, connection initially fails, but it eventually connects as an 802.11g connection (as shown by option-clicking on the network icon). When it does connect - not very well - it slows the connection of the macbook air way down.
I'm guessing that if I could force my wife's machine to connect in n mode, everything would be OK. Does anyone have any suggestions?
What I've tried so far:
.deleting and re-enabling the wireless drivers in the network preferences panel
.making a new network location
.deleting all relevant plists
.repairing disk permissions
Relevant background information:
.the 'n' router only supports channels 1-12 (i.e. we can't force n by using non-g channels)
.this network is in a crowded apartment block - I can see around a dozen other networks in istumbler
.the router is using directional paired (MIMO) antennas to bounce the signal from one apartment to another off an adjoining building. Of course, this could in general be problematic; but the received signal strength is OK - not good - according to both istumbler and a separate linux metering program. And the MBA works perfectly well in the same locations.
.it's using channel 1; there are two g networks in the building also using channel 1, but according to istumbler, our signal is strongest of the three in the 'receiving' apartment (in channels 6 and 11, there are other stronger signals, so 1 is the only rational choice).
.the router is running in mixed, not greenfields, mode
.although the MB is only able to connect in g mode, istumbler on the MB still shows it as an 'n' network
.we have never encountered any previous wifi problems with the MB (but this is the first time we've tried to connect it to an n router)
.and just to repeat, the router is using WPA2/AES encryption, which should be compatible with 802.11n
TIA for any help

Answer 1

SOLVED!
Who in the world would have guessed??
Trendnet support provided this information:
<<Note: Due to chipset compatibility issues, if you choose WEP, WPA or WPA2-TKIP encryption this device may operate in Legacy Wireless mode (802.11b/g). You may not get 802.11n performance as these forms of encryption are not supported by the 802.11n specification. >>
I had used WPS2-TKIP, just by accident. I changed it to Authentication WPA-Auto and Cypher type auto, and now it works on 802.11n, as well as g!! Even on my Powerbook G3 and HP laptop.
Who would have guessed?