Let's get units to be the same:
0.5 M stock = 0.5 moles/Liter x 1000 mmoles/mole = 500 mole/L = 500 mM stock
Now you can easily use M1V1 = M2V2:
(500 mM)(x ml) = (10 mM)(100 ml)
x = 2 ml of the original 0.5 M stock is needed to make 100 ml of 10 mM solution.
NOTE: The 500 ml of 0.5 M stock is not relevant as long as you have at least 2 ml, because that's what you need. Anything else is extra. Also note that the molar mass of Tris (121 g/mol) is not needed for the calculations. This is a simple dilution problem.