Re-installing OpenBSD

[stanl]

After fixing my /etc/mail/aliases file with jggimi's help, I continued to poke around where I shouldn't have been poking.
As a result I have to do a fresh install of OpenBSD but am stuck before I even begin.

I have a MacBook Air which I used to download the AMD64 version of install71.img from the OpenBSD web site.

Now, using the Mac:

diskutil list tells me the flash drive is /dev/disk2.

I unmount the drive with diskutil unmountDisk /dev/disk2.

Then:

dd if=/Users/stan/install71.img of=/dev/rdisk2s1 bs=512 (I tried bs=1M but got an error invalid numeric value).

After a few miunutes dd finishes without an error. I put the flash drive in my X220 and try to boot from it but the drive is blank.

What am I doing wrong?

Thank you