I think that you cannot use the SLM-128 plugin with Reaper.
According to Steinberg, the SLM-128 works only with Cubase 6.5 and Nuendo 5 or higher. Lower versions of Cubase and Nuendo may work, but the licenses of the higher versions are required anyway.
As far as I know, SLM-128 does not work in any audio production software other than stated above, not even in Wavelab. Besides, SLM-128 is a 32-bit only plugin (no 64-bit version available), and it has never been tested fully. See the related Steinberg news message of May 23, 2012:
https://www.steinberg.net/en/newsand...able-1970.html
If you are looking for a free loudness meter, try Martin Zuther's K-Meter:
http://www.mzuther.de/en/software/kmeter/
It may not look as polished as the SLM-128, but it works and is just as useful.