K550 is DB2020, you need to have a CID52 Phone to do unlock by Patch.
If you dont got, please use Seus (Sony Ericsson Update Service) first.
After Update you may have latest Firmware for CID52.

Now you can do unlock via Patch, by using Patch Generator and Setool2 lite 1.11.

I think instruction for that you find.

Sorry for my bad english :)