Ikea Zaventem by public transport? Stib site not working

STIB does not cover Zaventem so their route planner applicaton does not give instructions for it. You might have better luck with the De Lijn route planner.

Anyway, it's a complicated and lengthy trip, involving several changes of metro/bus and quite a bit of walking.

Have you thought about Ikea Anderlecht; it's a simple metro ride from Schuman.