Cannot find AppData under users file

Windows 7, To old to navigate Windows Explorer anymore....

Hi all, I'm sure this question has been asked a thousand times, but I can't find the info I need!
I'm trying to backup my "eM Client" data files, or complete e-mail backup, tech provides the following instructions; Backing up eM Client data files

You may wish to back up eM Client's data files, that include all the emails, calendar events, contacts, tasks and configurations. However, these data files can only be restored in eM Client.

You can find the data files in C:\Users\\AppData\Roaming\eM Client\ if you are using Vista or newer versions of Windows; or C:\Documents and Settings\\Application Data\eM Client\ if you are using XP or older versions of Windows.

The problem is, I cannotfind the "AppData" file under my user or public user file.
As well I don't understand the disappearing "My Documents" folder thing either, and this could explain why I'm having trouble navigating to the data files I need in the C: drive!
Thanks in advance for any help
N2

Thanks Danny for the quick reply!, I reviewed "Files & Folders" Options, and I already had " Hidden Files and Folders" ticked to show "Hidden files, folders and drives.
But was not able to find the application data folder.

So I un-ticked "Hide protected operating system files" Now the "Application Data Folder" shows up, but I can't get into it! to copy the file I need. ??
Any help here?
Thanks
N2

Danny, the error I get when trying to open App Data is this -C:\Users\All Users\Application Data is not accessible. Access is denied.

I went back to the tree in explorer to C:\Users and checked the permissions, All users are set to Full permissions.

If I find files that are not set to "full" and try to change them I get this error; An error occurred while applying security information to: C:\Users\All Users\Application Data

Thanks
N2