You may check for the hosting account details and access to the Plesk from the client area itself.
Steps:
1) Login to your client area
2) Proceed to -> Services -> My Services -> Choose the hosting and click on Manage Product and the nameservers details will be shown there.
By default the nameserver would be:
ns184.mschosting.com
ns185.mschosting.com
ns186.mschosting.com
Sample as below:
