Short and sweet: CLP.bat Login user pass[Enter] :: CLP.bat Post is posting from command line.

Also you can reply to the last plurk made using CLP.bat Reply Blahblahblah, or you can set the ReplyID if you know the reply ID in decimal.

Examining/Reading the batch file should provide clues to how it's used. If not, best to stick to the two basic uses (login and post)