how do I get itunes to show BPM for all songs?

I assume you've enabled it in View>View options. It can only show for songs where it's been entered in the Info by the originator of the file. iTunes can't generate the BPM itself.

McChris,

The BPM field can be viewed, but it is almost always empty unless you have taken steps to populate it.

You can use a 3rd party tool to perform BPM detection and add it to your iTunes database:

http://www.beatunes.com/itunes-automatic-bpm-detection.html