How would one best add the kelp extract? I have one litre of kelpak available for my IBC system.

This is a different type of seaweed than what you'd find in Seasol or Maxicrop so if I were you I'd go with a spray application using the concentrations listed on the label. You may have to adjust the volumes based on what I saw looking at their label. I'm not sure how the label on yours reads but the one I looked at gave volumes of kelpak diluted in 20 to 50 gallons and then spray applied to an acre of plants. Just mix smaller quantities but keep the ratio of Kelpak to water the same, then spray apply to the plants.

If you are using it just for adding trace elements and a bit of Potassium to an established system, and not for cycling, then people commonly use the Seasol brand available here in Oz, at 1x capful (about 15-20ml) per 500L of system water, per week, for leafy greens. If there are fruiting plants in the system, then 2x capfuls (30-40ml) per 500L, per week, does the job.

My kelpak arrived today. The label recommend 2-3l per hectare so I mixed up 0.5ml in 250ml of water and sprayed that quite liberally on the leaves of my plants in my growbed (and some others around the garden). I figure that should do the trick. I'll repeat again in 4 weeks