First off, MLS doesn't plan to fill up 50,000+ stadiums in the near future. It's first priority is building soccer-specific stadiums that hold 30,000 seats or under. They want an intimate environment for the matches.
As for making MLS more popular, there is only one thing you can control with: Viral marketing. MLS could spend more money on foreign players, increase roster size, pay the players more, eliminate the playoffs, get ESPN to broadcast more matches, etc... But these are decisions made by the league and each club for financial and competitive reasons.
If you want more interest in MLS, fans need to pass along the MLS message voluntarily. It's how web sites like Google got big. It wasn't through massive TV advertising but by word-of-mouth and clever Internet marketing. How did Barack Obama win the Democratic nomination over a more connected Hilary Clinton? MLS is an underdog compared to other team sports so viral marketing is essential to its success.
What message would you give to Americans on MLS soccer? It's not the best league in the world. It doesn't have the richest tradition. I can't think of a great message except MLS players love soccer because they are so underpaid compared to other sports.
To non-soccer fans, you have to answer: Why should they watch soccer? You can talk to them about the fans and their enthusiasm. You can tell them how anyone can be a soccer player if you have skill, no matter how big or tall you are.
Answering questions like these will help a little. Joining and organizing supporter groups helps bring fans together too.