I didn't do anything else, but you have to do exactly what I said. (Maybe you did.) Inside of c:\Users\Public\Public Documents\Overloud, there are three folders named REmatrix, REmatrix Player and REmatrix Solo. You have to go inside the one name REmatrix, not the others. Once there, you'll see a folder named IRLibraries and a file named userIRs.rir, but that is NOT the file to replace. It's almost as if Overloud was trying to make it confusing and difficult to do this. You have to go inside the folder named IRLibraries, where you'll see a file named REmatrix Solo.rir. Place your version of userIRs.rir alongside that, and it should work. At least, that is how it looked on my computer, and it is working.
Now, this does not create any new presets in Rematrix player, it just adds IRs. To see the IRs in Rematrix Player, you click the button below the slider for the Hall IR. This opens the extended panel for the Hall IR on the left hand side of the gui. The upper part of this panel now shows IR files instead of preset banks, starting with the category REmatrix SOLO. If you scroll down in that window, you will come to a category named My IRs, followed by whatever IRs you had previously imported into the REmatrix Solo file named userIRs.rir that you copied into the location described above. Double-clicking any of these will pop it into the Hall IR slot, and off you go! Note that the other four slots will similarly show a new category of MY IRs, but they will all be empty, so the only slot you can insert your user IRs is the Hall IR.
Hope this will make it work for you.