Download SCDToolz:
http://hoperomz.emucation.com/zip/SCDToolz350.zip
You can use it to not only identify and properly rename your Sega/Mega CD ISO, but also find out what region it is, and also make a cue file for it.
You said you have a Mega CD console, but you didn't say if it was Japanese or European.
Assuming it's European, then you need games with Europe for their region.
If your game is not for Europe, then you might be able to convert it's region to Europe with this:
http://www.retrodev.com/ConvSCD110.zip
Obviously, if your console is a Japanese Mega CD, then you need games for that region instead, and you might be able to patch Europe and US region games to Japan region using ConvSCD.