Just for listening, I would imagine just a nice shielded audio cable from an audio source on the 840 (speaker out?) would do the job - however, for transmitting, that's a whole 'nother ballgame. This site has some excellent information on interfaces - (note: all blue text are links)
WM2U's Soundcard Interfacing Schemes
It has a section on the 840, and in a downloadable PDF file, a schematic for what looks like a simple interface you can build. Or you can get the RigBlaster or any of the other pre made interfaces....
Anyway, it's always more helpful to have questions like this answered by users that use your equipment and software, so you can get hints that may not necessarily be in a user's manual...
ft840 : Yaesu FT-840
multipsk : multipsk