TB I believe he is asking how you install finalsun. Not what happens when the map file itself appears to be corrupt.
I am not sure what version you have downloaded but to install the newest version (version 1.01) its just a simple drag and drop. From there you click on the giant green icon that says FS on it. And wala FS opens up and asks for your language and tells you something then opens up.
However, if it is not allowing you to extract the contents from the ZIP file then perhaps you should try to redownload it from somewhere else. Where did you download it from last time?
Though if you have it installed and it is corrupting when you are attempting to open a map then the problem is what Team Black has described to you.
If you can provide more details about it or even pictures of the message and when it is occurring then that could help us help you.
Finally...welcome to the community Arkroyal4. I hope you enjoy your stay with us and I also hope to see something cool from you in the future.