am I translating this sentence to its symbolic from correctly? The ad goes "My old whitening cream didn't work. With (product) I got a lighter skin tone in two weeks! I am definitely sticking to (product)."
I came up with:
p: I use (product)
q: I got whiter skin
~p-->~q
p-->q

therefore p
Is this correct? Is the ad even an argument?
 A: The two assumptions you have just states that $p$ and $q$ are equivalent. To conclude that $p$ is desirable, you need to assert that $q$ is desirable, i.e. that lighter skin is something you want.
As a side note, there's also some real-world problems here, the most prominent being post hoc ergo propter hoc. You switched cream and noticed a difference. In real life you cannot conclude that the cream works just from this, since there are many other effects at work, such as how much exposure you get to sunlight, and what you eat. This is a major problem in for instance medicine, and it's the reason repeated doubly blinded, randomized trials are a necessity. Many who swear by alternative treatment are guilty of this logical fallacy, among others.
A: The ad can hardly be considered an instance of modus ponens.
A: Let's say you have 2 products $X$ and $Y$.
If $X$ is better, you will use $X$, but not $Y$: 
$XBetter \rightarrow UseX \land\lnot UseY$
If X is not better, you will use Y, but not X:. 
$\lnot XBetter \rightarrow UseY \land \lnot UseX$  
X is better:
$XBetter$
Therefore, you will use $X$, but not $Y$:
$UseX \land\lnot UseY$.
