It's not mSATA, it's PCI-e. You can maybe find an adapter here but it might not fit in your MacBook Pro, as the caddys are a little larger than standard SATA HDDs:
http://www.microsatacables.com/drive-adapters-and-drive-converters/apple-ssd-adapter
Out of interest, why did you buy a MBP 2015 SSD? If you put that in a 2012, it won't get anywhere near the 2GB/s read/write speeds as the 2012 has a 6Gb/s SATA III bottleneck, so you'd be better off returning it and buying a conventional SATA SSD.