Clicking on Products -> All Products menu at the left menu bar or All Products in the middle of dashboard page take you to the manage products page.
Choose the products you want to publish to the added marketplace channels by selecting check-boxes and click on menu Actions -> Push To Channel.
Now it will ask you to choose the channel where you want to publish the selected products. Choose a channel and click on Push button.
It will first verify product and channel specific data and submit the publish request if no error found otherwise it will show you the list of errors found during verification process. After publishing request selected products will be published on selected channels.