I recently downloaded Mugen and...I have tried setting up a all ready made character I tried using Winrar to extrat the information changing it to the required name and the character dosn't pop up I have the Beta Mugen on my comp

google it and you will get some documents on how to set up mugen

Remember that the folder name has to be exactly the same as the .def name found in the folder, try making sure the names are identical before trying again.