Jordi Gali optimal price setting
The third line of p.45 denotes: 

$MC_{t+k|t}=\psi_{t+k|t}/P_{t+k}$

but this is probably not the answer you are looking for.

Going from eq. (9) to (10), multipliy the right-hand side by $\frac{P_{t+k}}{P_{t+k}}=1$ (you can always multiply by one), the numerator goes into $\Pi_{t-1,t+k}$ (together with $P_{t-1}$), the denominator goes into $MC_{t+k|t}=\psi_{t+k|t}/P_{t+k}$.