Here's a brief overview of installing packages in a Debian based system:
Actual package installation is done by dpkg. This is a text based application. For an already downloaded package you could do 'dpkg -i <path & package name>. But this may fail because there may be dependencies which dpkg does not know how to find and has not been told to install.
So the next level is apt-get which will search the net for the packages it needs (including dependency packages), download and install them. It uses dpkg to do the actual install and maintains a file called /etc/apt/sources.list to know where to search for packages. This file must be updated regularly as packages get moved around in the internet repositories. This is done with the command 'apt-get update'.
But to use apt-get you must know what packages are available, so you will need a 'package browser list.
The most basic of these is dselect, which is text based and not overly user friendly. It also maintains a package available database file which must be updated regularly with 'dselect update'. I've been told that dselect update will update both the dselect database and the apt-get sources.list so it is preferable to use this instead of the 'apt-get update' command which only updates sources.list.
More user friendly alternatives for seaching for packages are the gui based applications Kpackage and Synaptic. Both of these provide information on the packages available and can download and install the package.
Dselect, Kpackage and Synaptic all use apt-get to do the actual downloading & installing.
Debian based systems must use either Debian packages or applications compiled from source. To install an RPM package, first convert it to a .deb using 'alien'.
ie: alien -d <rpm packagename>
So, for an already download package, you can try dpkg -i <package> but be prepared for dependency problems which you will have to solve yourself by manually tacking down and installing any required dependencies.
Better way is to use apt-get & let it redo the download:
First, you must become 'root'. Permissions are a big thing in Linux, only root (administrator level permissions) is allowed to install things. So, type 'su' to become root. Then do 'dselect update'. Usually the first time it is run it will need to download quite a bit & some of the downloads may fail. If this happens, just run the command again, eventually it will get all the data it needs.
then 'apt-get install <package>'. It should download the application & all required dependencies & install the application.
Selecting <package> from Kpackage & following the install procedure (check mark & install marked, etc) should do the same thing. But you do have to 'dselect update' first. Kpackage is going to ask you for a root password so you will have to set one before you run it. Open a terminal & type 'sudo passwd', then enter your new root password.
For more detailed information on the commands avaiable for apt-get & dpkg type 'man apt-get' or 'man dpkg' to view the manual pages. Also see the apt-get/dpkg tutorial post in the HDD install section of this forum.