Or you could use 12" to "whatever inches" of appropriately sized windshield washer hose and a pencil blower adapter (like in this link) to connect the two.
http://www.harborfreight.com/pocket-...ler-68262.html
The adapter you have is really limited in how to connect it to the compressor. It's designed to hold the air chuck against the crossed plastic molding inside the entry fitting.
For convenience, there are a number of other fittings that much better meet the need (and practicality) for actually "working by yourself" to blow out the lines.
The one I prefer is the one that connects to the compressor with a quick connect fitting like all my other air tools. Now, even though the fitting "lives with the water pump" in our RV, it's just a matter of attaching it to the city water inlet, connecting the air hose and turning on the compressor (REGULATED TO 30 PSI).