Based on a little poking around and this thread, I think
apt-get install mongodb or
mongodb-10gen will "just work" and install a 32-bit mongo server on a 32-bit linux distro, or a 64-bit mongo server on a 64-bit distro.
Which do you have? Try this:
That tells me I'm running 64-bit linux. So I can choose to run either 32-bit mongo or 64-bit mongo. If there were a "32" in there, I'd be running a 32-bit linux distro, and could only run 32-bit mongo atop it.
Now, take all this with a grain of salt, as I personally use
apt-get for practically everything except mongodb, which I manage several versions of concurrently.
So, I find it simpler and more predictable to just go to the 10gen distributions page, and download and untar the version I want. After that, executing the right
bin/mongod just works right out of the box.
Hope this helps!