You have to download the same version, same language, same program to use the given Serial.
Eg. If you have Master Collection US-EN CS6 you have to download exactly that version, but not another version, another Type like Design Collection or a single program, same language, same OS.
It sounds like you have an older perpetual license to InDesign. If you tell us what version, we could provide you with a link to download the software. Of course, you'll need your serial number to authenticate it.