Graduate school needs my masters grade but I have no grades yet!
The best way to do this would be to insert "n/a" into the field for your grades, or simply write "not available yet" as CaptainEmacs suggested. In the event that you cannot write a non-numerical answer (as was the case when I applied to grad school), a "0" should suffice that it is not your actual grade. Your application will likely require preliminary transcripts (and eventually official transcripts), so I would not worry about not being able to report your official grade, especially since this is a common problem.

I would avoid writing this issue into your statement of purpose, since your SoP should be relatively short and to the point, focusing on your research and other accomplishments. While I would definitely include your MSc course in your statement, I would not mention anything relating to the application itself. Often universities have a graduate coordinator for any given department, so I suggest reaching out to them if you have questions about your application.