$ sudo apt-get update
$ sudo apt-get install drupal6-mod-xmlsitemap 

drupal6-mod-xmlsitemap is installed in your system.

Make ensure the drupal6-mod-xmlsitemap package were installed using the commands given below,

$ sudo dpkg-query -l | grep drupal6-mod-xmlsitemap *

You will get with drupal6-mod-xmlsitemap package name, version, architecture and description in a table

drupal6-mod-xmlsitemap package basic information: