Here's how I would do problem #2:

M_{initial}V_{initial} = M_{final}V_{final}

M_{initial} = x

V_{initial} = 100µL = 0.1mL

M_{final} = 5mM

V_{final} = 5mL

(note: Instead of using M1V1 = M2V2, I find that it is easier to work M_{initial}V_{initial} = M_{final}V_{final}. 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.