Could someone please tell me where I can find the Portal 2 Sixense MotionPack DLC for Razer Hydra to actually DOWNLOAD it? Supposedly it's only on Steam, but if I go to the page here:

http://store.steampowered.com/app/660/?snr=1_7_7_151_150_1

There doesn't seem to be any button ANYWHERE to actually download it! I bought a Razer Hydra and it's supposed to include a free copy of this version of Portal 2, but I have to download it.

try: steam://install/660

All that does is redirects me to my search engine and tries to search for that text, which returns results to the steam website but not that page.

You may also try downloading it manually. Run steam with -console parameter.
in steam console window type/:

A quick google search turns up this:

Razer Hydra Portal 2 bundle
If you purchased the Razer Hydra Portal 2 bundle, you can find a key code in the box. Enter that key code into Steam->Game->Activate a Product on Steam... This code will activate a copy of Portal 2 and the MotionPack DLC

[Razer Hydra without the Portal 2 bundle
Some Razer Hydras are now offered without the Portal 2 bundle for a reduced price. If you have a Razer Hydra and already own Portal 2, send an email to p2motionpack@sixense.com to request a Portal 2 MotionPack DLC code. Include the serial number from the bottom of the Hydra base in your request. It's the numbers and letters following "S/N"