The first step was to add memory, which requires disassembling the machine almost completely to pull the motherboard out. After that, I moved the included 250GB hard drive from one of the 4 drive bays to the 5.25" bay using a 3.5" to 5.25" adapter.
I got a license from VMware and downloaded a version of ESXi with HP drivers from this page. I used vSphere 5.1 U1 Apr 2013, which was really this image: HP Custom Image for ESXi 5.1.0 Update 1 Install CD.
I "burned" the iso to a usb stick using unetbootin-windows-584. I put the stick in the server and booted. It promptly complained about the media failing a check. I formatted the stick and ran unetbootin as administrator. This time, the microserver booted to a VMware ESXi 5.1.0 Installation.
At this point, I also had a USB stick inserted on the motherboard to install to, in addition to the 250GB hard drive. I planned to install ESXi to the USB stick, and use the 250GB for VMs. I'm going to add 4 hard drives later and try to add them to the NAS via Raw Device Mapping (RDM).
The install took a while, but completed with a few inputs from me (disk, location, verification, etc.). I removed the install media and rebooted to a 60 day trial version. Unfortunately, I had not connected the server to the network, so the IP address was 0.0.0.0, then 169.254.241.82. It noted that DHCP failed. I plugged in a cable, and it picked up an address in a minute or two.
I connected from my workstation to the address listed. It pointed me to various VMware tools, including the vSphere client. I already had the current version: VMware-viclient-all-5.1.0-1064113.exe.
I installed the client and logged in using the IP address and root user/password. I had to verify/allow a security certificate.
After logging in, I saw a note that the license would expire in 60 days. There was a link, which took me to the VMware site, where I logged in. I then saw that I had a license key for a 60 day evaluation. It took me a while to figure out how to update the license, which I had requested earlier for evaluation.
To add the license, go to the Home view, then select Inventory. This should show the host. on the right, go to Software, then click on "Licensed Features." It should show 60 days.
There is an "Edit..." link in the top right. Click on that, then add the key.
I created a datastore using the 250GB disk, and then I was off making VMs!