Download the setup file from the Movie File Merger Website. Basically, you just have to put the installation folder somewhere, where you have write access. Don't put it in the Program Files folder.
Working with Dropbox is part of the current design of the program. However, the program can also work standalone, like from a memory stick or normal folder on the hard disk without Dropbox. Double clicking on the program in its folder or starting it from a shortcut is all you need to do run it.
MFM runs under Win 7, Win 8, Win and 10. MFM has also been used on a Mac with Parallels installed.