Hi, welcome to KH. Is this
your pattern? There are instructions for a set up row under Garter Tab Cast On and before Section 1
Set Up Row [WS]: K3, yo, p1, pm, p1, pm, p1, yo, k3.
that include "pm" or place marker. If the pm instruction is ever omitted, I think it would be a fairly safe bet that it should be in the spot where you're instructed to slip marker.
That's a cool shawl and looks like a fun knit. Enjoy! We'd love to see how it goes for you, we just love pictures.
BTW it isn't a dumb question, these things can be notoriously hard to spot. I find they're usually hidden in plain sight when someone else points them out to me.