For Mac User:

One of the easiest way is by changing the file name, for instance, the video file I have now is named "cempaka.wma", "wma" means "Windows Media Video", it can't be played by "Quicktime Player". To change it you must first unhide the extension by selecting the file, hit "Ctrl-I", it's under "Name & Extension". Now you should see the extension, you just rename the file by removing the ".wma" and change it to ".avi".

***This is the lousiest method, but if you do not have internet connection, you may try this, no harm trying it, anything goes wrong, just rename the file to what it was before.***

You can also download some convertor which could convert your video format to your desired video format in a proper way (unlike changing name). There are many video convertor in the internet, choose the one you feel like the best, go to "Google" and search for "Video Convertor For Mac".

Besides that, download video player is also another easy and safe way to play video files. For example, you have a .rmvb video file, you can download "Real Player" from REAL. I would suggest a very cool player which can play almost every video file, it is the VLC player.

There is also another way which is you need to download plugins for your player. I don't know how to explain the word "plugin", maybe I should tell a short story. A team represents a "Video Player", and the team members represent "Plugins". So, if a team only has 2 members, then there are many things that cannot be done, however, if the team has more members, then many things could be done easily. In another words, you need more "plugins" for you "video player" so that it can play most video files. Another term for "plugins" is "codec". To get one of those plugins, you can also go "Google" and search for it.

I'm so tired, can I stop here? (This is not an option, please say yes, I mean you have to say "YES")