I just now booted the game for the first time, quit it, then booted it a second time and then downloaded both ME1 DLC installer files to my PC from the link in this guide (EA Classic DLC download page) and then I tried installing them both.

I had already performed the community-suggested REGEDIT inspection and changes since two string values were not there upon my inspection.

So, I added the universal CD key as a string value in the ".../.../...Electronic Arts/Electronic Arts" registry folder under the "ergc" string and I also added the string: "Flavour" with value: "Steam" in the other registry folder ".../.../.../mass effect TM/".

And, sadly, the two installers cancelled the installation on me every time. I tried pasting the universal CD key for ME1 into the DLC installers, but only the second DLC installer would even let me.

Unfortunately, the universal ME1 CD key does not work and the official Steam CD key generator tool for this game does not work at all right now.

So, I think I'm just not going to be able to play these games before the Legendary Edition comes out, which really sucks.

I was able to install both DLCs by editing "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Electronic Arts\Electronic Arts\Mass Effect (tm)\ergc". Modify "(Default)" key and set value to "5GUCXRFPQNBMTJBD3L5J" (taken from https://help.ea.com/en-us/help/faq/dlc-for-classic-games/).

Execute both installers, providing the same key for Pinnacle Station. Remember to do this before running ME1 as it changes the activation code back to the default value.

I actually got it to work with your help, Eyes3x, but only after I ran the two official EA DLC installer programs with administrator privileges. That was the last piece of the puzzle I had to discover before they would successfully install the DLC.

It's resolved for me now so, thanks very much to everyone in this thread for all the information and support!

Now I just need to figure out why the game freezes for one second every 6 seconds while in-game. :)