Terry is spot on as usual. By default we'll configure the appropriate RAID setup based on the number of drives you have within 100MB of the first detected disk. If you go into partitioning you should be able to see the custom setup which has been defined, but there shouldn't be any need to change anything.
If you're having issues with the graphical installer, try the text installer. It's not super friendly but you only have to set time zone and then hit 'b' to begin.
Once installed, if you have any issues with RAID, share the contents of /root/sme-partitioning.log to help us debug.