Google results for "iTunes Match download all" weren't fantastic, so once I figured it out on my own, I decided to document it here for future generations.

The problem: I have two Macs, and a lot of the music on one was not yet synced over to the other via iTunes Match. While it was easy enough to download one track at a time by clicking on the iCloud icon beside the track, that was going to be a huge pain for a large number of songs.

Here's what I did to solve this.

Make sure that the "iCloud Download" column is visible in iTunes. You can enable this via the iTunes menu with View->View Options->iCloud Download
You should now have an iCloud column visible when looking at your music library. Click on the cloud icon to "sort by iCloud"
You should now have all the songs available via iCloud (but not yet on your computer) grouped together in your library. Simply select them all (click on the first one, scroll down, hold "Shift" and click on the last one), right-click or control-click (hold the "control" button and then click) on the list of selected songs, and choose "Download".

That's it! All your iTunes Match songs should start to download.

Yes, in hindsight this is all quite obvious, but I spent a good five minutes looking through iTunes for a button that says "Download All".