Actually this isn't all that hard.You don't even need to do any soldering to do this.
Search for and download a proggie out of the Netherlands called PDW. It's a good little ACARS monitor. Next, grab a scanner and get a standard stereo patch cord with 1/8" mini plugs. Connect one end to the earphone jack of your scanner, the other to your mic input on your computer, set PDW to decode from your soundcard, tune your scanner to an ACARS frequency and away you go.
Now, decoding pagers in the US is illegal. So if you live in the US I must insist that you do NOT choose Flex/Pocsag as the protocol being monitored, and DO NOT search between 929 and 931 MHz on your scanner. If you do YOU WILL ILLEGALLY RECEIVE PAGER COMMS! YOU HAVE BEEN WARNED!
I trust everyone here to do the right thing.
-AZ