I think the plunger is missing and/or don't know how to set up the Pressure Gauge components for use.
Your Pressure Gauge system is comprised of three parts: The stainless steel plunger, stem, and the gauge itself.
The plunger arrives already inserted into the brew cylinder. Simply use your combination dosing cup/tamper to push it up to the top of the brew cylinder (side opposite of the red o-rings) for use. You will do this after each shot to reset the plunger, as well.
Your gauge will arrive already installed on the stem. For use, simply "plug" the stem into the plunger, after you've filled your brew cylinder with your brew water.
The gauge should be on your left-hand side, and you can rotate it slightly for optimal viewing.
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If you've removed your gauge from the stem (for o-ring replacement, storage, etc), simply screw it back onto the stem afterward. You will want to leave 0-1 threads showing: Do not over-tighten, as you will risk damaging/displacing the o-ring in the stem pocket.