Question: I am using KDE 4.8.2 and I want to know if there is a simple way to go from the search bar, in the start up menu, to the results. It's really annoying not being able to just press the down key and get to the result.

Example:
I press Alt + F1, I start type what I want, what I want appears but to access I have to use the mouse. How not to use the mouse?

Answer: 
I have no Mac to check, but on Unix there are generally some more commands to show who is logged in:

* who (you already mentioned)
* w (with detailed information)
* users (you also tried; just shows names of currently logged-in users)
* finger (with some personal information)

So maybe one of the not-yet-tried (i.e. w, finger) does its job correctly for you? If not, some more possibilities especially for OSX are listed in the article [osx - How to know when and which user logged into the system](https://unix.stackexchange.com/questions/14127/how-to-know-when-and-which-user-logged-into-the-system-under-mac-os-x-last-is-n).
On ubuntu you do `last` and you can see all ssh and non ssh logins. I don't have mac here, I will try when I get back. Bug quick google search tells me it works on mac too.