To install libxt6 just follow these instructions.
sudo apt-get install libxt6
Which will install libxt6 and any other packages on which it depends.
|Maintainer||Debian X Strike Force <firstname.lastname@example.org>|
|Description||X11 toolkit intrinsics library which other toolkits are based. Xt is the basis for many toolkits, including the Athena widgets (Xaw), and LessTif (a Motif implementation).|