I purchased this as an accessory for The AR390SS I just Purchased. There is no way of using these extension wands with the AR390SS.
Sheldon Does Not Recommend This Product
I am sorry you're having issues with the extension wands. The page for the AR390SS says that you'll need an adapter to use the rest of the Blue Clean accessories (PW3082750) and I see from your order that you also purchased the adapter.
When you wrote to us on 3/11 asking how to use the adapter, we responded with "Thanks for the inquiry. You will remove the current threaded lance on your unit. You would screw the adapter in where the current lance was. This would make it a bayonet connection in which you would attach the extension wand to. Any questions, don't hesitate to give us a call at 800-710-7499." If you didn't see our response it might be in your junk mail folder.
I hope this helps! We appreciate your business, and apologize for any inconvenience.
Extension kit is designed to work with our fixed brush attachments. It is not intended to use as a HP lance extension. Call 866-235-5112 8-5pm M-F if you need more information on how to use this extension kit with your 390SS.