To load the scripts:
1. Start Reaper
2. Press F4 for the actions dialogue
3. Tab until you get to the New Action button
4. Press shift space and a context menu appears.  Arrow down to Load Script.
5. An open dialogue appears where you can browse and select a LUA script file

To use the script:
1. Put a number of different recordings of the same thing onto a track.  For example, 5 recordings of a drum being hit.
2. Select all the items you want to put into round robin in RS5K.  You can select one item, press control alt A to select the remaining items on the track.
3. Open up the action dialogue with F4 and search for the action.  I type RS5K robin.
4. A dialogue appears.  Choose the midi note you want to use.
5. The script will click and whirr and place an RS5K instance on the track for each selected item.

Now press the note on your midi keyboard you chose.  It will play one of the items.  Press it again and it will play a different item.  It will keep playing a different item until they have all been played once and then start again.

You can hear what effect this has by first going through these steps with just one item and then playing it repeatedly quite fast.

Then do it with 3 or more items.  It should sound much more natural.