Couldn't be simpler, but it took me a little while to figure it out too. Open CMP and look in the lower left corner. There are grey tabs for the various action panels. Select the one that says "Uploads". Drag and drop your file from the Main list into that action panel, the same as you would for a download. A new window will pop up asking you to "Choose what assets..." and your item will appear in the list with a checked box to its left. UNCHECK the box. If you leave it checked it will attempt to upload all the dependancies your item uses, and then it will tell you you can't do that because they're not your assets. (I figured all this out the hard way). Once you have all that, simply click the "Upload" box at the bottom of the action panel.
