Here's how I would do problem #2:
MinitialVinitial = MfinalVfinal
Minitial = x
Vinitial = 100µL = 0.1mL
Mfinal = 5mM
Vfinal = 5mL
(note: Instead of using M1V1 = M2V2, I find that it is easier to work MinitialVinitial = MfinalVfinal. That way, it's harder to confuse which volumes go with which molarities).
Therefore: (0.1mL)x = (5mM)(5mL)
x = (5mM)(5mL)/(0.1mL) = 250mM
g/dl means grams per deciliter. A deciliter is 1/10 of a liter = 0.1L = 100mL
So, probably the easiest way of thinking about it is that:
1g/dL = 1g/100mL
NB: g/dL is equivalent to (w/v)%, if you are familiar with that measure of concentration.