Excel -- Converting Decimals to Commas in order to calculate.

Hi!

How can something so simple prove to be so difficult? I've been trying to solve how to convert decimal points from imported excel files to commas so I can make some calculations. 

I've searched on line and the instructions I find are as follows: File > Options > Advanced (see attached file). I'm running Excel for Mac 2016. The thing is when I click on File I do not see an option to drill down to an Options function at all see file nr 2.

Grateful for some help