Briefly:

There are several steps involved in setting up an assay to measure compound inhibitor potency. Since you are using a kit, you don't need to worry about choosing the buffer conditions or the substrate concentration. Since the enzyme does not come with the kit, you will need to choose the enzyme concentration. Measure product formation versus reaction time at various enzyme concentrations. Choose an enzyme concentration and reaction time that maintains a constant reaction rate the whole time (i.e. linear progress curve) and gives a sufficient signal. It is not necessary to convert the absorbance signal to urea concentration for this purpose, so you don't need to make a standard curve.

Prepare a set of serial dilutions of the compound covering the range of concentrations over which inhibition occurs. Measure the signal at each concentration. Plot a curve of percent inhibition versus compound concentration. Use nonlinear regression to fit the data to a suitable IC50 equation, such as the following (the simplest one): % inhibition = (100 x [I])/(IC50 + [I]), where [I] is the inhibitor concentration. The IC50 is the concentration at which the reaction rate is reduced by 50%. It is a typical way of measuring compound potency.

Without knowing the concentration of arginine in the assay or the Km for arginine under assay conditions, you can't calculate the inhibition constant (Ki).