For być and its conjugates to have instrumental, there must be two nouns (substantives) in the sentence, one the subject and one the objet. for example, "to jest rower" (It is a bicycle - only one noun, the object),compare with Joanna jest polką (Joanna is a polish woman, two nouns, subject - Joanna, object - Polka). (Many thanks to Joanna, my Polish teacher in Kraków who explained this to me.
Joanna is right, but there's one detail missing: subject pronouns are also valid here. "Ona jest Polką" = "She is a Polish woman" :)
Oh, and we do capitalize the demonyms for countries and continents, so that's why it's "Polka". Although we don't do that with demonyms for cities, so "Joanna jest krakowianką" (if she really is from Kraków, of course).
In "Ta jest kobietą", Ta is the subject and kobietą is the object. In "Gdzie jest kobieta", kobieta is the subject and there is no object. Gdzie isn't a noun, so it can't be a subject or object of a verb; Ta is a pronoun, so it can.
The subject of "jest" should be in the nominative case (kobieta) and the object should be in the instrumental case (kobietą).