There are a few steps involved.
If you want to load Absynth and Battery into Reaper as VSTi, you have to:
1 Make sure that Reaper is pointing to the folders in which those VSTis and their DLL files sit. In Reaper, go to "options" on the menu bar, then down to "preferences". This will bring up a Reaper Preferences panel. On the left hand side, towards the bottom, you will see "Plug-ins", and under that "VST". Clicking on this brings up another panel which allows you to specify where Reaper has to look for the plug-ins.
2 To use, say, Absynth, in a track, just create a new track (CTRL T), click on FX, click on VSTI, then scroll through the various plug-ins until you find Absynth, then click on that. Absynth, starting with "A", should be near the top.
3 To hear anything, you need to create (or record) a midi item within this new track. Quick way: go to "Insert", then "new midi item". Double-click on the midi item just created, add some notes at random in that scroll view, then hit play.