help with a new screen

I won a 3.5 huge New in box screen on ebay lucky me

its metal frame with black velour edge

I have assembled it with a lot of carefull thinking and planning


now it has slotted end plates for mounting and the screws they supplied are smaller than the slots for mounting to the wall . that puzzles me , the only thing I can think of is these plates must be a in a axact aligement to the screws on the wall ... so how does one mark them out


The next is on the screen surface there is a small dot of what looks like black grease

Please whats the procedure to get this off without leaving a mark

my email is [email protected]

any assistance would be appreciated