Update: The page appears to have been fixed. I had emailed HTC support reporting the error, and they replied to me within 24 hours saying the page had been fixed. I am very impressed with their efficiency. Hats off to you HTC.
It's pretty simple really. You use wget to get the files directly, setting the referer to the link itself. Here is the exact command. Wait for the files to download. To ensure that the files are the right ones, you may want to do a checksum.
Once you get the files, go ahead and follow the instructions on the page mentioned above. They worked fine for me. However, I have read about a few people having problems with adb not recognizing the device. Keep in mind that you don't have to go through adb as mentioned on the HTC page. You just need to get the relevant updates onto the SD card with the name of update.zip - you can do this with Windows Explorer, Finder, Nautilus (or whatever it is you use) if you prefer doing it that way.
If you're having trouble with adb, try this instead:
Mount the phone's SD card onto your computer somehow (either through the USB cable, or by plugging it in directly). Once you've done that, copy the updates (one by one, as mentioned in the tutorial) on to the SD card, and rename it to update.zip (irrespective of what update it is). Then follow the instructions on the HTC page. This hopefully should circumvent any adb problems.
That should be all you need to get Android 1.5 onto your ADP. Have fun with it!