I got the same error 'System.Windows.Forms.MessageBox' is a 'type' but is used like a 'variable', even if using:
MessageBox.Show("Hello, World!");
I guess my initial attempts with invalid syntax caused some kind of bug and I ended up fixing it by adding a space between "MessageBox.Show" and the brackets ():
MessageBox.Show ("Hello, World!");
Now using the original syntax without the extra space works again:
MessageBox.Show("Hello, World!");