Ok, i'll take it slow... :P
First, you get the file, right?
It's a tbz2 so extract it. The .a extension doesn't mean anything. It confused File-roller as well.
Look at the .a file as a .raw file.
But for this, to mount it, you need a Linux installation with mount. You do have that, right?
These are the commands:
That will mount it, and expose all the files in the image. So, let's get the files. The folders are hidden, so you want to show them. (In xterm, it doesn't matter. Just use ls -a.)
Most of the folders are self-explanatory. the .overlay-* folders contain some data. Dunno what it's for, though. (Possibly for using a "mount --bind" command.)
Search for the .squashfs-files. They contain the OSs, and you can easily mount them without selecting filesystem. (Of course, you need loop.)
Like this:
Most preferably you'd extract the images from the file before mounting.
The squashfs files aren't directly usable by N900. They need to be put over into a new container, like a .img.ext3 file. (I did so.)
That will make them usable for chroots and etc.
What more do you need to know about the process?

Sadly, my N900 isn't up for chroots... It's stuck with browerd atm... -.-'