Jordi Gali optimal price setting Monetary Policy, Inflation,
and the Business Cycle, 
Jordi Gali,
Page 45
http://elib.peaceland.edu.ng:8383/greenstone3/sites/localsite/collect/peacelan/index/assoc/HASH0139.dir/doc.pdf

How does "ψ " convert to "MC"?

You can say Drago Bergholt solutions for Gali
http://bergholt.weebly.com/uploads/1/1/8/4/11843961/the_basic_new_keynesian_model_-_drago_bergholt.pdf
but Drago also didn't explain this change. (page 13 )

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