If it is legal to do so (and I can only assume you have the key via legitimate means), then the only two options are:
1. Assuming RAS isn't used, buy the same DMR equipment that's being used and then add the key to this to decrypt, or
2. If (and only if) the Motorola Basic Privacy flavor of encryption is being used, then acquire an ADCR25 receiver and it will decrypt it without the need for a key. Note you can no longer buy an ADCR25: they were withdrawn from sale shortly after some simple decryption features were added....I'm sure you can draw your own conclusions as to why it's no longer for sale !