Because it is not a native Neo Geo ROM, you will not find a standard .neo file. Instead, you are looking for an or a Converted Neo Geo 4.0 ROM .
Here are the steps to download and play Metal Slug 6 Rom: NEW- Download Neo Geo Roms Metal Slug 6 Rom
This is the necessary legal disclaimer. SNK Playmore (now SNK) still holds the copyright for Metal Slug 6 . The game was re-released legally on: Because it is not a native Neo Geo