You could just hydrate it with water and introduce it to your system. The manufacturer does not give hard numbers for an appropriate inoculum size for hydroponic systems. If yours is relatively small (< 50 gallons), 2 grams should be sufficient. Otherwise, you can purchase and inoculate a larger amount or grow your own from a small inoculum into a nutrient broth. Honey or syrup works well, as @tyler274 suggested, but I like to use light dry malt extract (DME, used prodominantly in home brewing). Find the lightest you can, because this is uncaramelized and has the highest nutrient content compared to those that have been heated/caramelized. I mix DME at 20 grams per liter. I will typically pressure cook the broth to ensure it’s sterile, but it shouldn’t typically be an issue as long as you at least boil it. Make sure to allow it to cool to room temperature while sealed, before inoculation a small anount (less than 1 gram). Tin foil works well for this. Once inoculated, I seal it with a tight-sealing screw-on lid. Stirring helps, but if you don’t have a magnetic stirrer, just let it sit and periodically shake it vigorously. The skating helps to break up the nodules that form (they look like small spheres when grown in broth on an orbital shaker). Breaking up these nodules promotes faster growth and prevents the nodules from getting too large, which may allow them to settle to the bottom of your reservoir or get trapped in the plumbing. After 24 to 48 hours at room temperature, your culture should have doubled in size several times. It’s difficult to determine the actual biomass unless you filter and weigh it, but I wouldn’t let it go longer than 3 days growing without having performed strict sterilization and aseptic techniques to ensure your inoculum is likely the only thing growing.