What does 3/5 and 5/5 mean?

I assume you mean the activity today. In the options, you can configure the maximum counted posts for the day. Each day, you can get credit for visiting, and making each of the configured posts. The first number is how many credits the user has earned today, and the second number is the maximum they could earn. Note, however, that the visit credit can be weighted, so 1/5 does not necessarily mean 20% activity today.

Great, thanks a lot :)

Im wondering would this hack slow down my entire site? My forum has around 97k of members, and would 1 query per page per member slow down the page a lot?

With only the default options enabled, this mod should be imperceptible, except when you first install it, and arround the midnight you configure for the mod, if you place it at a time that has a lot of load to start with (which is one reason why the mod's midnight is separately configurable). Once you start enabling extras, however, there is potential for slowdown. As you say, enabling "Find today's activity for all users", which can add up to 25 queries to a thread page could slow it down, which is why it starts disabled.

How do I Add it to "Quick Links" ?

The stats?
Add
to the navbar template where you want, probably after the online.php link section.

Thanks a lot. I really appreciate your help :)

I using 2.71 version on my 3.6.4, would like to update, but what to update???

Just import the new XML, making sure to check the overwrite checkbox. Unless you see a reason to update templates in the change log, you don't need to.