So presumably you've created an 8-disk RAID 5 right? (btw we're not big fans of R5, preferring R6) but in this situation we'll let it slip as it's such a small array).
That said it looks like you've successfully created the array and a logical disk - so there's no reason you shouldn't be able to install on it so long as whatever OS you want to use has drivers for the P420i you've got. Basically you've done good so far, you just need to get the drivers installed. Best thing to do is go HERE and download the appropriate drivers for your OS (might as well grab the BIOS updates as well while you're there), you should then be able to add them during the installation process.
Come back to us if you have further details or want to go through the installation process, we will need to know what OS you want though ok :)
Oh and that's a nice server by the way, I'm a big HP buyer and they're great.
EDIT - thanks for the extra detail - as discussed ESX/i 3.5 isn't supported on Gen8 servers unfortunately, you'll need to use version 4.1 or later. If you download the HP specific version of the latest version it'll have all the lovely monitoring and instrumentation goodies included too.