I have investigated this a bit and found that the Fn+Sym is opened when the the Multi_key is pressed. But there are a number of keyboard mappings floating around that don't map this key. If you have this problem I guess that you are using a localized keyboard. You will have to modify this mapping to add back the mapping of the Multi_key. Be very careful when playing around with keyboard mappings and preferably install open-ssh server so that you can connect externally to your N900. Otherwise you may find yourself without any way of typing on and fixing bugs.

Personally I think it is a big mistake that maemo "grabbed" the Multi-Key for the popup. The Multi-key is traditionally used in X11 for the Xcompose mechanism. (See http://canonical.org/~kragen/setting-up-keyboard.html). The popup window is not as extendable. E.g. if I press "Multi_key - >" I get the forward arrow → . There is no easy way to input that under maemo.

Another problem with the popup window is that it only works with touch screen and thus you have to move you hands away for the keyboard.

There might also be a problem with using multi-key with external keyboards, but I'm not sure as I haven't tested it.