OK, after that your map files and folders should look like this image.
You will need to delete the zip files client & server that you just extracted. I highlighted them in blue in the image.
Then go to each folder , right click on it, choose "properties" , uncheck the box that says "read only" select "appy" , then it will ask you if it should do the same for applying changes to folder,subfolders and files. Select then OK.
look at all the files to see if they need to be unchecked for read only so that changes can be made to them.
It appears that Editor folder may already be in that map. Based on my extracted level. In there you will see all the CON files that were used in editing the original map. GamePlayObject con file is in there for a 64 player but not sure if it is coop,cq or sp. So You should replace it with whatever game type you are creating. Go to GameModes and choose what version you will be making. Looks like they are all 64 player.
I would suggest you rename your level at this point to whatever you want. Use the link that Coronaextra has for you for renaming your level. At this point it is ready to be renamed.
MFC157.tmp , archive.md5 , mapdata.py - are files that are also in your extracted map. I'm hoping someone here could give you more information on those files and if they can be used in your new map or not. As I am not exactly sure what they do.
Now your map should be ready to be loaded in editor. But if you are going to change the terrian shape and overgrowth/undergrowth you will need to follow the steps in that second link that Coronaextra gave you. What it will do is change those dds images to the proper format for editing. You might be spending some time there with trial and error like I did before I figured it out.
But, If you are not planning to change the terrian you will not need to do this step.