>>>I have put the code
>>> IF WEXIST("frmBuchhilfe")
>>> _Screen.frmBuchhilfe.Show()
>>> ENDIF
>
>Unknown member frmBuchhilfe. I get the same error if I use _screen.buchhilf.show().

frmBuchHilfe is not a member of _screen; but as you test it with Wexist() you can extend the logic of using windows instead of their forms, so
IF WEXIST("frmBuchhilfe")
activate window frmBuchHilfe
ENDIF

...so much fuss around one help window :)