Upvote:3
You are a German citizen, lawfully living in Canada whilst attending a university course. Assuming you never overstayed your visa in the US, I am pretty sure you can enter USA on the Visa Waiver Program as a German Citizen. Since you are entering by land, you will have to fill in the I-94W form, and cannot apply for an ESTA for this trip:
If you are a citizen of a country that participates in the Visa Waiver Program (VWP), and you enter the U.S. by land from Mexico or Canada, you are only required to complete the paper I-94W form at the land border crossing. ESTA is currently required for air and cruise ship travel only.
Bring all the supporting information you can use to demonstrate that you are lawfully residing in canada and are genuinely travelling to the US for the sole purpose of attending the conference. This includes, and is not limited to: