Are you recieving errors?
There is a couple of things to be aware of.
Content Button
Click on the ADD CONTENT button.
Leave Type set to Email, select the email you want to add.
Stream + Icon
Click on the streams + icon, leave Type as Email and select the email you want to add.
Drag & Drop
Simply drag the email from the tree into the stream.(Your email needs to be approved to add to a stream.)
Check out this article for more help: http://community.marketo.com/MarketoArticle?id=kA050000000LAW4CAO