I saw a few people asking about this and some saying they successfully installed it, but didn;t see anyone explain exactly how they did it, so here's how I did it:
I was able to get this installed on my Apple TV 1 (with aTVFlash Silver) by doing the PleXBMC install via ZIP file. There were a couple tricks along the way.
1) Download the appropriate pleXBMC add-on for your version of XMBC on your AppleTV (11.0/Eden in my case),http://wiki.xbmc.org/index.php?title=Add-on:PleXBMC
2) Transfer the ZIP file you downloaded to the Apple TV. I used FTP from my Mac to copy the 'plugin.video.plexbmc-v2.1.zip' file over to the Documents folder on the AppleTV,http://support.firecore.com/entries/259290-Transferring-Files-via-FTP
3) Then I launched XBMC on the Apple TV and followed the ZIP add-on install proceedure from the XBMC online manual, http://wiki.xbmc.org/index.php?title=HOW-TO:Install_an_Add-on_from_a_zip_file. I found the ZIP file I had transferred in steo 2 on /Home/Documents on the Apple TV
4) Once installed I was able to go to Videos > Add-on in the XBMC interface and access pleXBMC.
5) Now the tricky part. I couldn't figure out how to access the settings. It seems to try and come up on first launch, but closes immediately. To access it, on the Videos > Add-ons screen, with pleXMBC selected, press and hold the menu button on your remote. That will bring up the settings. Then you can go in and configure the pleXMBC client to set Bonjour discovery and add in your myPlex username and password. Once that is done then you should be able to see and access your Plex Server.