First off, Paessler WMI Tester sounds useless, so I would recommend uninstalling it. Server 2012 already has a veritable cornucopia of WMI tools that it ships with, so there's no need to install yet another "WMI tester."
Consider the following, from your Win8 desktop:
PS C:\> $Options = New-CimSessionOption -Protocol Dcom
PS C:\> $CimSession = New-CimSession -ComputerName 'Server2012' -SessionOption $Options
PS C:\> Get-CimInstance -CimSession $CimSession -ClassName Win32_OperatingSystem
Add whatever you need to those options, such as credentials with which to access your server, if needed. If your server and your workstation are in separate domains or if you're running Powershell from an unprivileged account, you will need credentials.
You shouldn't even need to get that fancy.
PS C:\> Get-CimInstance -ComputerName 'Server2012' -ClassName Win32_OperatingSystem
Should be sufficient. The first example is more complex, but is capable of targeting down-level machines that do not even have Powershell installed. Which your Server 2012 machine does. Powershell Remoting is turned on by default in Server 2012. But that's another topic...
If you are still getting
RPC Unavailable errors, try simply
telnet server2012 135 ... does that telnet work? If it fails, then you are looking at a firewall problem.