Problem 1 is that you probably need some dependencies and the install files for the application(s).

If I look at this list, then I do see quite a few.
Note: I don't even know if this is the application you're referring to, and/or if this is the correct version.
Seems the link I found is for

You'll also find the .deb file there, and through the listed dependencies, you may be able to find then .deb files of those as well.
Scroll all the way down "Download", where I found the link: 
http://ftp.de.debian.org/debian/pool/main/n/network-manager-applet/network-manager-gnome_1.20.0-3_amd64.deb

Obviously this is the wrong CPU version, so I looked here: http://ftp.de.debian.org/debian/pool/main/n/network-manager/
Where I found more files that you may need. 

You'll have to do the same for the needed dependencies that are not yet installed on your Pi.
You could try apt-get to see if these are installed:

apt list --installed

Maybe combined with grep to find something specific:

apt list | grep something

Not sure if "apt" is available on your system though, and since you're running Debian, I'd assume you have dpkg available:

sudo dpkg-query -l

Here we could use grep as well of course:

sudo dpkg-query -l | grep something

Problem 2 is installing things.

You have two ways of going around that.
I suppose the "official" way would be to install the dependencies first and then the network manager stuff.

Or ... option 2 (not recommended): install the network manager stuff and run it and see what goes wrong and install files when needed.
The latter is obviously not the best approach.

Problem 3 would be what tool to use to install.

Since you on Debian, I would assume (not being an expert) that installing a .deb (Debian) package would work like so:

sudo dpkg -i <path of .deb package/packages>

Or something like this:

cd <location of folder where .deb files are> 
sudo dpkg -i *.deb 

Hope this helps ...