First, here's the New York Times headline on the CPAC conference: "GOP Divisions Fester at Conservative Retreat." Festering an infected wound, it's awful. I guarantee, if there were a liberal conclave comparable to this, and there were vigorous debates going on there, the New York Times headline would be, "Healthy Diversity Flourishes at Liberal Conclave."
While one could argue what event would qualify as a liberal/left alternative to CPAC, it's safe to say that none of them get as much media attention as CPAC.
But there are such conferences. In D.C., the liberal Campaign for America's Future organizes one. Back in 2006, when it was called "Take Back America," this was the New York Times headline (6/14/06):
Clinton and Kerry Show Democratic Divide on Troop Withdrawal
And here's another headline (6/8/10) about the same conference (though it had changed its name):
Pushing a Liberal Agenda, With Democrats as Target
To be fair to Will, his other point–that political media needlessly portray ideological conflict within parties or movements as some kind of problem–has some merit.
But the idea that one headline about a lavishly over-covered conservative conference that features appearances by Sarah Palin and Donald Trump is evidence of liberal media bias–that's just a reminder that there's just not much of a case to be made here.