Which bag, 250mL or 1500mL? For the big bag, remove the plastic strip that allows the two tabs to lift up, then remove the plastic cap on the outlet. Slide into the pro adapter, then screw the red cap clockwise, that's it. Prime your doser (manual dose until the liquid comes out.
When inserted into the adapter, the tabs (wings) are lifted up, which allows the liquid/gel to pass. The pic above shows the security tab removed from the top portion, but the bottom port still has the cap on it, which gets removed before you insert into the adapter.