uboot not seeing Mer on sdcard in N900:

I followed the instructions here to put Mer on an sdcard. I have uboot on my n900 from an earlier time when I was looking at Nitdroid on an sdcard. It's very important to me not to brick the phone or blow up my Maemo on the phone itself. As with Nitdroid, I just want to take a look at Mer and see how I like it.

(I tried to ask my question in the Mer threads but was directed here. I'm not sure I belong here either -- -- so apologies if that's the case!)

When I power up with the slider open, I get the uboot screen, but my choices are Maemo5 (which works fine), or "Nitdroid" even though the sdcard has Mer on it.

I'm obviously missing something, but the more threads I read the more confused I'm getting! What do I need to do to get uboot to see my Mer card? (Edit: I did read the first post and few pages in this thread, but didn't understand it well enough to apply it to my case.)