I got both a converted machine to Lion and a new one with Lion.
I use Wifi connection
1. Connect the 8500a to the Wifi network
2. Leave it on
3. Go to printers and add a printer
4. It should show your printer
5. Once selected it will ask you to install the drivers
6. Say yes
7. Ready to go
Scanning
1. Use Preview
2. Include Network scanners
3. Select your printer
Method 2:
You may check if you have the latest drivers installed for the device. You may refer to the below link for getting the latest drivers and check HP Officejet 8500A Driver