Phoo,
Persevere with ReaSurround... it's not that bad once you get your head around it! But I'll come to that in a sec.
As for your hardware setup, it sounds as though you've got that set up correctly based on what you've said. Let's step through this one bit at a time....
First up. Your audio track with the mono source on it. That track needs to be set to 6 channels (via the track count drop down menu in the I/O panel).
Now add an instance of ReaSurround. To begin with, see the drop down menu which says "stereo"? Open that up and select one of the 5.1 options. I personally use the ITU-R spec. When you do this, the number of input channels will automatically increase to 6. We don't need that. Change it back to one. You'll see that there is now only 1 puck in the GUI instead of 6. Also, 5 of the 6 channel input faders at the bottom left will disappear. So the input faders allow you to control how much signal from your audio track is feeding into the surround panner. The OUTPUT faders, as the name suggests, allow you to manually override what goes out of each channel, separately to your placement of the puck. Don't want signal going to the Ls Rs even though the puck is moving away from the front? Drop the output faders for the Ls Rs channels.
The trap for new players though, in terms of LFE, is that the dropdown menu at the bottom labelled "input faders" also doubles as your LFE send. Play with it. You'll figure it out if you've done this stuff before.
The "edit selected inputs" section has a whole bunch of different parameters you can automate. You can choose which 3 you want to designate to the 3 available controllers via the separate dropdown menus. The rotary controllers are infinite, but for them to work, your puck must be 'selected'. Simply click on it with your mouse and it'll grow wings. That lets you know it's selected. Not selected = no wings.
OK, so that's setting up ReaSurround.
Get out of there and back to your tracklay.
If your audio track is routed to a parent/folder, that parent/folder also needs to be set to 6 channel output (via the I/O panel), but does not require a ReaSurround plugin on it, as all the 6 channel routing has been done at the track level. Make sense?
Also, your master output needs to be set to 6 channel output.
You should now be rockin'.
And if you want to get your mind totally blown...
Reaper's unique "pin architecture" for plugins means you can load multiple instances of your favourite stereo compressor, but assign different inputs and outputs! Freakin' awesome or what? Love a Waves SSL compressor? Load it up 4 times on your channel/parent/master... in the first instance, leave the input/output pins assignment at its default. That will handle compression on 1/2 (Lf Rf). Go to the second instance of your compressor, bring up the pin controller, select inputs 3/3 and outputs 3/3. That compressor is now compressing just your centre channel. On your 3rd compressor, set the pins to in 4/4 and out 4/4. THAT compressor is just compressing your LFE channel. On the 4th compressor, set the pins to in 5/6 and out 5/6. And that compressor is handling your Ls Rs channels.
Sweet as honey.
Sing out if you're still stuck.