Unlike Windows, the default user in *NIX, is NOT an admin. You can't just install software like in windows. It is a bit more locked down, and the way of doing things will take some getting used to.
In Mint, to install software:
In a terminal (Cmd window in Windows):
You can use apt-get to install a program (it has quite a few command line options) for nasm, in a terminal you would type: "sudo apt-get install nasm" without the quotes, then hit return.
You can use either the package manager or software manager, both will require the root password. just type in nasm in the search box, hit enter, then install.
You could also install NASM from source and compile it.
You will most likely need a few more devel packages for all to work. I think it is " build-essential" so you would "sudo apt-get install build-essential"
If you have trouble with anything, MAN is your friend.. just do "MAN something" to get the manual - "man apt-get"