You mentioned an on-screen keyboard and a *.MER file, which suggests that you are using a touchscreen and FactoryTak View ME.
You're also referring to the ordinary path to the Task Manager on the C: drive with Windows.
Are you running FactoryTalk View ME on a PanelView Plus hardware, or an ordinary Windows 7 or Windows 10 PC ?
Do you have a physical keyboard attached that will let you use Control-Alt-Escape ?
In my quick-and-dirty test running FactoryTalk View ME 11 on Windows 7 32-bit, I was able to click a Program Launcher ActiveX button with just the string "C:\Windows\System32\Taskmgr.exe" as the Program Location, and the Task Manager appeared.
I haven't done this on Windows CE, like the PanelView Plus terminal. If that's your hardware, post some details about the terminal's age and generation (PV+6, PV+7, FTView ME revision, etc).