I need sendmail on my server,
I'm sure you don't. What makes you think you do?
but how do I install it on a 7.0 pre1?
You'd need to first uninstall qmail, which would break all the email features of the SME server. But qmail is a much more secure, reliable and tractable MTA than sendmail. It would be unwise for you to take such a large step backwards.