To install xnest just follow these instructions.
sudo apt-get install xnest
Which will install xnest and any other packages on which it depends.
|Maintainer||Debian X Strike Force <firstname.lastname@example.org>|
|Description||Nested X server X server, where it runs as a client. This means that it appears as another window in your current X session. Xnest relies upon its parent X server for font services.|