Thanks kabamaru, this was very helpfull.
I had to use pluma rather than gedit (I think pluma is the default text editor in this version of Mint), so I typed:
gksudo pluma /etc/apt/sources.list
I then changed the sources.list file as you suggested, and success followed:
magic@magic ~ $ sudo apt-get source htop
Reading package lists... Done
Building dependency tree
Reading state information... Done
Need to get 395 kB of source archives.
Get:1 http://archive.ubuntu.com/ubuntu/ precise/universe htop 1.0.1-1 (dsc) [1,112 B]
Get:2 http://archive.ubuntu.com/ubuntu/ precise/universe htop 1.0.1-1 (tar) [385 kB]
Get:3 http://archive.ubuntu.com/ubuntu/ precise/universe htop 1.0.1-1 (diff) [9,113 B]
Fetched 395 kB in 0s (1,221 kB/s)
gpgv: Signature made Sun 19 Feb 2012 08:29:27 GMT using DSA key ID C154998C
gpgv: Can't check signature: public key not found
dpkg-source: warning: failed to verify signature on ./htop_1.0.1-1.dsc
dpkg-source: info: extracting htop in htop-1.0.1
dpkg-source: info: unpacking htop_1.0.1.orig.tar.gz
dpkg-source: info: applying htop_1.0.1-1.diff.gz
My question now is: Where do I find the source code that I've just downloaded?