When you downloaded them they were probably in a compressed file format, like .zip or .rmf, so extract the .mdl file or files from that.
They'll probably be named the character that they either are, or are going to replace, such as goku.mdl, or ssjgoku.mdl.
You'll probably want to make backups of the models you have before you replace them with the ones you downloaded, so you might want to do that.
Once you do, just copy the .mdl files you downloaded into the correct folder located in esf\models\player, so goku.mdl would go in esf\models\player\goku, gohan would go in esf\models\player\gohan, etc.