I have found out that myAppointment.RTFBody appeared with Outlook 2010.
The problem is that I don't know how to use it.
I copied an example from http://msdn.microsoft.com/en-us/library/aa287595(v=vs.71).aspx

myAppointment.RTFBody = @"{\rtf1\ansi This is in \b bold\b0.}";

unfortunately whatever I put after myAppointment.RTFBody, I receive an error:

System.Runtime.InteropServices.COMException (0xB9B0FFFF): The operation failed.

Can yo please help me with creating proper appointment with BOLD text in it?

THX in advance.

I heared that RTFBody need some binary stuff to put in...Anybody heared about it?

Did you read the documentation on that page? It says, in case you didn't, that you can get the String from the bytes by using System.Text.Encoding.AsciiEncoding. Did you read about that class? Did you notice the GetBytes method?