The default package management tool used by SL5 & SL6 is YUM.
In addition to your OS repositories, if you would like to install EMI, UMD, gLite or IGI products you have to proper set up the correct repositories.
For a successful installation, you will need to configure your package manager to reference a number of repositories, specific for each distribution.
You will find mirrors for all of them hosted in Padova, as primary mirror on http://repo-pd.italiangrid.it/mrepo/, and CNAF, as secondary one on http://repo-cnaf.italiangrid.it/mrepo/
Here you can find all the links to the official INFNGRID repo files: http://repo-pd.italiangrid.it/mrepo/repos/ for all profiles.
Example file: igi.repo
# # INFNGRID repositories #