First I had to ensure that the stock debian packages are available for installation:

$ tail /etc/apt/sources.list
deb http://archive.debian.org/debian etch main

Install g++ and other dependencies

$ apt-get install g++ libpam0g-dev

Download google-authenticator source code:

$ git clone https://code.google.com/p/google-authenticator/
$ cd google-authenticator/
$ ./configure && make && make install

Then simply follow the instructions over at here