I had the same problem, and I could not get it to work by specifying the name of the shelf. However, I found that if you delete the shelf name altogether, TFS will display a window with the selected list of available shelves to choose from. Select the desired regiment and perform all other merge operations as normal.
Example: c: [mapped workspace target path]> tfpt unshelve / migrate / source: "$ / Sourcepath" / target: "$ / targetpath"
source share