Mac 10.8.3 Admin name and password

Hi all,
i have a strange issue with my MacBook Pro 13.
It started when some of the application didn't work. For example, i use MplayerX to view video files. Everything was ok, but now if i double click a video file, the MplayerX icon jumps on the dock, and thats it, and it won't play.

Another weird thing - i can't edit or create playlists in iTunes. Started on the same time.

Now, when i try to install apps not from Appstore, i get the app_name.pkg can’t be opened because it was not downloaded from the Mac App Store.
I am trying to allow installation of outside apps, on System Preferences -> Security & Privacy. When im trying to open the make changes lock - i'm being promot to enter admin name & pass. Usually the Name field is already with the name, but now it's empty. I don't know my admin name, and i tried my regular name and pass, those taht i use to login to mac - didn't help.

Note - under User & Groups, i see my regular user, the current on, with the full name. I tried to enter this name in the admin name field, and also the name of my user as it appears on /Users/user_mac folder - no luck.

What are my admin credentials, or how to fix this issue? Please advise, thanks.

@ justperry,

as i said, a big part of my problem, is that i don't know my admin name & pass.

Weaselboy said:

The user_name part at /Users/user_name is what you will want to enter in any login box.

Click to expand...

Right i know, i believe that was always written there, but now suddenly the field is blank, and when i type my /Users/user_name there, and the correct pass it fails to authenticate.

Weaselboy said:

In the Users & Groups pane under your name does it say "Admin"? Are there other accounts listed there also?

Click to expand...

No it says - Standard.
There is another account - Guest User - Sharing only, and its grey and not selectable.

OK, this was a tough one.

@ Weaselboy - i followed the tutorial and created a temp admin account, and logged in with it. Surprisingly, this account was also privileged as 'Standard' and i couldn't do any changes with it.

Then, i decided to re-install my OS, and so i did, via the Restart+CMD+R screen, but after that i still remained with the 2 unprivileged accounts and no admin.

Finally - i went to the disk utility and completely erased my Macintosh-HD drive and then re-installed the Mac-OS. Now all ok, i have admin, but need to install all apps from the beginning

Thanks anyway, this has been a big pain in my...
Why would an admin account will disappear like that?