On the left hand side of the window, you'll see multiple tabs, click the input/output tab, it should then show this window on proguard
On the right hand side of the input/output tab, you'll find several other tabs, we'll only be using the add input and add output tabs so dont worry about the rest
In this window, we will be inputting your .jar's location and outputting where the obfuscated version of your .jar would be going to
So for example, my .jar is currently on my desktop named DivineFate.jar
We would click on the add input tab which brings the window to search your computer, find where your created .jar is saved
After that, click the add output tab, specify what you would want to save it as, in my case ill be saving it as ClientObfuscated.jar and then select the destination where you would like the obfuscated version of your .jar to save to, i'll be saving it to my desktop as an example
After you've added the input and output, you'll be finished with the input/output tab of proguard.