I have 100mg of Protein Kinase, 30u/mg, and I need to make a stock solution that has a concentration of 100ug/ml.

My question is, how much PBS do I need to add to my PK in order to get that concentration?

30u/mg, can you please check it again
what is it?
30units per mg

30u/mg. I took that directly off the bottle. I imagine it means "units."

okay
concentration = mass / volume
now we have
mass = 100mg = 100mg X 10^6ug / 10^3mg = 10^5ug
volume = ? we want to calculate (stock solution)
Concentration= 100ug/ ml
volume = mass / concentration
=   10^5ug/ 100ug/ml = 1000ml
you need to add 1000ml of PBS to make a stock solution 100ug/ml