The issue is that 32 bit installations are not compatible with the C FREEDV RADE decoder. 
I re-installed OpenWebRX+ as 64bit and RADEL/RADEU was included by default. 

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I am running openwebrx+ on my Debian box (AMD64) and I was able to compile the software into a .deb file, then install the deb.  I'm not related to the project at all, but I'll try to help if I can.  

 

At https://github.com/peterbmarks/radae_decoder  I followed the instructions under "Install dependencies (Debian/Ubuntu)".  I then followed the "quick build" under  "Debian Package for webrx_rade_decode" .

 

A .deb file was built, then I used dpkg to install it.

 

It installed a file /usr/bin/webrx_rade_decode

 

I restarted openwebrx+ and boom!  there were new modes listed as "RADEU" and "RADEL".  I tried using it a bit on what I thought were FreeDV signals and it did not seem to be working.  No sound.  But maybe the signal was too weak or somthing because as I was scanning the bands last night, I ran across the Tuesday night FreeDV net and it worked flawlessly.

 

I don't know what platform you're trying to run openwebrx+ on, but it looks like the RADE decoder page has good instructions for MAC also.  If on a Raspberry PI, then I'm guessing the Debian instructions MAY work on there.  I don't know.  I hope this is of some help.