Ok, first things first. That link points to a repository, a place where pieces of software (usually called packages) are. The way they work is the following.
You first tell your system that this is a repository you want to use.
Then, you can view and install packages from that repository.
You can do those things in at least two ways, one involving the console, and one involving the graphical interface of YaST. Let’s try that.
So, first thing is, start YaST, by using the menu launched by the green geeko at the bottom left part of your screen (then type yast in the search).
You will be asked for your root password. Type it in to proceed. Off course, that means that YaST is granted root priviledges and if you don’t know what you are doing you can can screw your system. No worries though, just follow through carefully.
Secondly, click on ‘Software Repositories’. This is the menu that shows you (and allows the configuration of) the repositories your system uses.
Select ‘Specify URL…’ and click ‘Next’
For the ‘Repository Name’ you can type in anything you want, for example ‘openSUSE games’
For the URL, paste the link Index of /repositories/games/openSUSE_12.1 (it is the one you found)
And click Next.
You should be now on the previous window, click OK. On the message, click Trust.
You have now successfully added the repo in your system.
From YaST (should already be open in the background) you can open ‘Manage Software’ and install anything you want.
To specifically see packages in your newly added repo, click ‘View’ on the top left corner and select ‘Repositories’. Then click on ‘Games’ (or the name you specified in the repository name step previously).
You can now tick for installation anything you want. Note that NOT every package you see there is a game (or in general an individual piece of software).
Another way is to click on ‘View’ and select ‘Package Groups’, then click on Games (that is NOT exactly the same ‘Games’ you added generally speaking, meaning this is not the repository), and select what you want from there. Descriptions are very helpful.
Good luck, and feel free to ask anything. There are no questions that shouldn’t be asked.