Log into your MnG account and click on "Upload Photos". You will see the form shown above.
Your photo must be on your computer, in a subdirectory. I don't think you can upload it directly from Lightroom.
Photos must be in one of four formats. If your photo is in any other format, your file will not be uploaded. (I think it just sits there.)
Set Photo Properties
If you click "Set Photo Properties" you will be able to change the Title, Author, and add a description of the photo to upload.
Click "Select" to start selecting files to upload. A Directory or Finder window will appear and you can navigate to the place where your photo is stored on your computer. Once a file is chosen, you can Select another file to upload the same way. If you change your mind, you can remove a file from the upload queue by clicking on the red "X".
When you have selected all of the photos you want to upload, click "Start Upload" and the files will be uploaded to your Album.
Select an album
Sorry, we won't let you modify anyone else's Album. You should always see your own name in the album box. If not, let us know!
Alternate Browser Uploader
If you have trouble using Flash on your computer, you might want to try using (click "Browser uploader") the Browser uploader instead of the Flash loader described here.