Media 7.0-2.x and Plupload
It's been a little while for me and it was tricky at the time, but enabling these modules should help:


Multiple Forms
Media Multiselect (provides a "Media Multiselect" widget instead of the standard Media Browser)


There was an additional patch to Media and a plethora of other issues (mostly related to permissions and editing files) that I noted as resolved as of 2014-Sep.

As you point out in the comments, a patch to the Media Multiselect module was still required to get the most recent dev versions of Plupload and Media 2.x working together by changing the widget type to that provided by Media Multiselect: 

Patch (required for now but under review): Issue #2216273: Latest media changes break the multiselect (Comment #52)

Cheers -