By default, you should be able to view the print-friendly version of a MediaWiki page by clicking on the Printable version button located in the toolbox section on the left side of the wiki (assuming the default skin is used).

However, if the button is missing, or you don't want to use it, you can simply add this string to the page's URL:

&printable=yes

and you will see the print-friendly version of the MediaWiki page.

For example, if you wish to view the print-friendly version of:

http://yourdomain.com/wiki/index.php?title=Main_Page

you should add &printable=yes to the URL so that in your browser the address is:

http://yourdomain.com/wiki/index.php?title=Main_Page&printable=yes