You should add namespace if you are not using it:
System.Windows.Forms.MessageBox.Show("Some text", "Some title",
System.Windows.Forms.MessageBoxButtons.OK,
System.Windows.Forms.MessageBoxIcon.Error);
Alternatively, you can add at the begining of your file:
using System.Windows.Forms
and then use (as stated in previous answers):
MessageBox.Show("Some text", "Some title",
MessageBoxButtons.OK, MessageBoxIcon.Error);