"Nightmares Unleashed" in Dungeon is mine. No Quori, sorry.
"Class Acts: Warden" in Dragon is also mine.
These will be my first published works in either of the online magazines, although I've done a handful of pieces for the DDI free content side.
Ways to get published:
1. As stated above, pitch, pitch, pitch. Sometimes it just takes persistance.
2. Read, read, read. Go over the past six months of both magazines and see what they've hit on lately. Don't pitch those things. The exception is Class Acts, since it seems like they're hitting all of the classes pretty regularly. It's a bit of a crap shoot, cuz you never know what they've got in the works currently. I've pitched ideas that showed up in a magazine just a couple months later, meaning that the project was well underway when I pitched mine.
3. I know you may not want to hear this, but knowing the right people can be very helpful. Network and establish name recognition. I got my start knowing ONE guy in the RPGA. I wrote a little for the RPGA. He then recommended me to another person. I wrote a little there. And so on.
A cold pitch coming from an unknown might be awesome and get the gig. But a cold pitch coming from a recognized name (even if only barely recognized) almost certainly gives the pitch a little extra consideration based on reputation.
It takes time, but is worth the effort to build up your reputation and establish good professioinal working relationships. I'm attending GenCon this year for the first time in four years. While I'm certainly going to enjoy the gaming and seminars and showroom, I'm gonna spend some time schmoozing and networking. And you don't have to do this at GenCon. Go to a local LFR Con and meet your local LFR peeps.
If you are friends or even acquaintences with someone who has written for WotC (whom you trust not to steal an idea), pitch some ideas to them and see if you spark their interest. You could get your foot in the door by co-authoring with them.
Work your foot in the door where ever you can and keep at it.