To install the SwarmPlayer video plugin (based on HTML5) visit P2P-Next's SwarmPlayer Site. You'll find a page such as the one in the screenshot below.
Find the green button labeled "Add Swarmplayer ...". Click it and follow the instructions.
To install the NextSharePC video plugin (based on VLC) follow the steps below:
... Windows, run the Windows installer.
... Mac, run the Mac OS X installer.