Hi,
the MediaWiki version number is in the file includes/DefaultSettings.php on your webserver. Somewhere at the beginning of this file there is a variable definition: $wgVersion = ... That's the version you have.
Before you are doing any of the below it might be wise to make sure that you have a backup of your MediaWiki installation.
On that page in the small boxes you see what the blog author typed on the command line. The word "nano" in front of "Namespace.php" is the command to open the file Namespace.php with the programme "nano", which is an editor. If you want to proceed with what the blog author does, you can just open this file with an editor and modify it as he writes: Go to line 46, change the following from:
class Namespace {
to:
class MWNamespace {
Then you would still have to take the other steps, which he has there. There he is basically replacing all occurences of the word "Namespace" with "MWNamespace" inside the "includes" directory.
However, according to the error you get, it is caused by your MediaWiki version being to old. Your version does not work with PHP 5.3. Most probably this MediaWiki version also has known security holes, which you should fix anyway. So it would also make sense to do an update of MediaWiki, instead of fixing all these errors by hand. However, this might lead you to getting even more errors in the time when you try to get it running again with a new version. But it would bring you on the save side security wise.