1 Download the latest SDR#
2 Download the EXT/IO add on from here SDR#_ExtIO_v1.1.zip http://www.meteorgis.space/files/ExtIO/
3 Extract both zips into a new folder (SDR# and the EXT/IO add on.
4 Place your RSP1a EXT/IO dll file into your newly created folder.
Follow the read me inside the SDR#_ExtIO_v1.1.zip file.