Usually the .deb’s documentation on its website or man page explains how to use that tool. But if it doesn’t, see below…
Whonix is based on Debian. Installing debs with “dpkg -i” is correct. .debs can not be started in that sense. They are supposed to be installed, what you already did. The question you’re currently having is how to use the package.
Graphical applications usually install a .desktop file, which you can find in start menu. You can check, if the package you installed contains a .desktop file.
Taken from source.
To see all the files the package installed onto your system, do this:
dpkg-query -L <package_name>
To see the files a .deb file will install
dpkg-query -c <package_name.deb>
If so, you should be able to start it in start menu.
When it doesn’t provide a .desktop file, and if it is a package that provides startable files**, it will most likely have installed that tool into a directory which is included in the $PATH variable.
To see the contents of $PATH, you can use this.
For example, if you had installed a packages called faketime… You’d see by using…
dpkg -L faketime
That it installed /usr/bin/faketime. So from now you can use.
(**There are packages that doesn’t provide startable files, but other stuff, such as files used by other packages (extensions), programming libraries, fonts, pictures, etc.)