iMessage issues between iPhone, iPad and Mac

2 questions both imessage related.

So I amended my telephone number through my apple and happily use iMessage on my iPhone 5. I have recently bought an iMac and have tried to sign in with my apple ID, which worked however when I message nfrom the mac it shows my email address. Under preferences it does not give me the option to set it to show my mobile number - can you tell me how to do this?

Secondly my iPad was previously set to an old number and I don't know how to change it - can you help me with this too please?

Thank you

Sue