From the few multimeters I have played with that had a com port on them, they only responded (sent) data after you sent them a command of some sort. You could either configure an arduino (with an RS-232 converter on it) and a LCD for example to display the data being sent or you could wrap the one com port running their software around to a second port running Putty etc. to see what is being sent to start the conversion. I remember one meter I had (still have, but no idea where the book is for it) included some simple BASIC code samples of the strings to send to get data from the meter. You might search for "BASIC multimeter commands" or something like that - it may be a standard set of commands.