I saw your PM and thought I'd find the thread instead of replying. Input on the bottom, output facing up works for me but.....
The trick to priming is supplying air-free liquid to the inlet and the easiest possible path to purge air out of the outlet. It sounds easier than it is in practice because you'll typically try to get started with both hoses hooked to things. (side note, you don't really need a valve on the inlet side if you have valves on the output of all your vessels.)
Let's say the pump is bone dry and so are your hoses. The input hose comes from your HLT and the output goes to your MLT. Open the pump output ball valve all the way, open the HLT valve all the way. If there's enough head pressure in your HLT, water should flow down and into the pump head driving air up into the outlet hose. Wait a minute or so for the air to work out. Turn the pump on. There's likely to be some trapped air in the head for a minute or two but it should catch and eventually drive out any air bubbles.
My other tip is to pre-prime the pump with your garden hose. I made up a disconnect to hose thread system so I can attach the hose to the input of the pump easily. With the valve open, I push high pressure hose water through until the output is solid water. Close the valve, disconnect the hose, hook up your vessels and away you go.
Maintaining prime is key. Watch to make sure you don't pull air in by shutting the pump output valve just before air gets to the pump housing.