That SoftPaq pre-dates even x86 64 XP release by a couple of years so I'm not surprised it doesn't support W7 64. If there are later 64 bit versions of HPs Easy Access Button software they may not support your keyboard anyway.

Well, in the driver files you'll find ia64 sections, so the driver is supposed to support XP 64Bit.
I don't know if it works on a 64Bit XP but it certainly doesn't on Windows 7.
I was just hoping there would be at least a Vista 64 Bit driver since it was still sold in the mid to late 2000s.

But I guess I'm out of luck.

That's for Itanium-based XP 64 which is a different platform and software base to x86 64 XP.

If Windows outputs virtual key codes for those keys, perhaps you can get it to work. You can use CharProbe to see what the keyboard transmits, and HoeKey to bind the scan codes to functions. I've used this approach in a similar situation, although in that case not all keys output anything Windows understood.

I don't know exactly what to do.
When I press one of the buttons the charprobe input field turns green temporarily, so I guess it means it recognizes something.
But I don't know what to do next.
Apart from telling me that "there was a problem in your settings" HoeKey doesn't do much.

I was able to get at least 4 (of the 8 ) buttons to work thanks to an extension of activaid
(the extension is called userhotkeys and is part of activaid)

https://www.heise.de/download/product/activaid-24593