Normally we would access NDE at http://managementIP of any storage node *before* anything has been configured (either storage or compute or Management Node) and deploy HCI that way:
http://docs.netapp.com/hci/topic/com.netapp.doc.hci-ude-17P1/GUID-8CED8C44-68D4-4AFD-9DB8-F611914AA637.html?cp=2_0_2_0
But wIth NDE you could not install only storage nodes, for example, because a minimum NetApp HCI cluster currently consists of 4 storage + 2 compute nodes. If you have the h/w but decide to install a storage cluster first, that'd be a "SolidFire" storage cluster. It is easier to install a "full HCI" cluster than separately.
If storage-ony cluster has been built, you'd use HCI Compute Node ISO that corresponds to ISO used to buid HCI Storage Cluster. For example if you used storage ISO from HCI 1.7.0, you'd get compute ISO from HCI 1.7.0 as well. This comes with Element (SolidFire) v11.5.
Newest HCI version is 1.7.1 which contains Element (SolidFire) v11.7:
https://mysupport.netapp.com/products/p/hci.html
When you get Compute Node ISO for HCI that corresponds to storage OS you got, assuming you backed up compute nodes' data you want to keep, you'd install Compute Node ISO which would do 2 things: refresh compute nodes' BIOS and firmware (if necessary), and install Bootstrap OS on the first internal SATADOM.
After a reboot, these nodes would be ready to deploy with NDE, which runs on storage nodes and deploys ESXi on the second internal SATADOM on compute node.
As I mentioned in top paragraphs, storage-only clusters are easy to install but if you already have all the h/w it's best to install from scratch using NDE.
1) HCI v1.7.1 would require management node VM to be installed and configured. Then you could go to:
https://[management node IP]/mnode
NDE installs Management VM for you, but if your existing storage cluster doesn't have it, you may install and configure it on your own, which may be harder than to just use NDE to install HCI from scratch.
2) HCI v1.6 and earlier (maybe even 1.7.1, I haven't tried), try HTTP management IP of any storage node
http://[any-storage-node-management-ip]
This "easy scale" function will let you add compute nodes only if they're properly cabled, VLANs and MTUs correct, etc. so while it requires just a few clicks (https://youtu.be/krWod7ZaBqE) to complete, network preparation requires the understanding of HCI networking options and other details (http://docs.netapp.com/hci/topic/com.netapp.doc.hci-ude-17P1/GUID-67E1EEF6-900D-4C7A-B1CD-3598A000E7FD.html?cp=2_0_2_5).
Personally if I had a system with no data to keep, I'd wipe everything clean and install using Compute and Storage ISOs from HCI v1.7.1.
Using 4 USB sticks: yes, I meant it saves you time to have 4 sticks for 4 storage nodes (4.5GB per image = you can use USB keys 8GB or larger). Compute nodes ISOs are getting close to 20GB and for those a fast USB 3.0 key moved from one node to another one may be better than several slow USB keys in parallel.
USB cables or kesy need to be slim enough to not get pushed by VGA cable which can cause read errors.