Got a solution :)
Hope it helps anybody.
Code:
tell application "OmniPlan"
activate
set xString to "Neuer Task 2" -- uniqe name of my task
tell front document
-- set xoutput to title of it
-- set foundTask to (every task whose name is xString) -- many results possible
set foundTask to (first task whose name is xString) -- only one result
set xoutput to completed of foundTask
-- set completed of foundTask to 1.0 --set to 100%
end tell
--set xoutput to completed of document 1
tell front window
set selected tasks to foundTask
end tell
end tell
xoutput