hello, this sentence is confused me 

onu görmek beni çok mutlu etti

is it wrong to say

onun görmesi beni çok mutlu etti

Theyre different things, if you mean "seeing him made me so happy", its correct " onu görmek beni çok mutlu etti" you re glad to see him

But if you say "onun görmesi beni çok mutlu etti", So, he saw and youre happy whatever he saw something

thank you very much, I have another question if we say

onun görmesini istiyorum.  görmesini does not mean whatever he see but it means I want to see him is this because the accusative "i" ?

thanks

Now I´m confused, I thought this would mean something likee "I want him to see"

Onun görmesini istiyorum.

This sentence means "I want him to see something(hidden object)" or "He is blind and i want him to see his around with his eyes again"

For example;

Onun [evi] görmesini istiyorum.

I want him to see [house].

Onun [beni] görmesini istiyorum.

I want him to see [me].

See the use of -me/ma as an infinitive: http://www.turkishclass.com/forumTitle_61847

onun görmesini istiyorum (I want him to see)

görme + si = his seeing