Here is a link to a lengthy and frightening analysis of the EMP threat at Arutz Sheva. How close to, or far off, the mark is it? Beats me. However, even if we knew for a certainty that it would happen on April 10th, there would few steps, if any, to take now to prevent it; without at least relative certainty we seem unlikely to do anything useful. After such an attack? Maybe nuke Pyongyang, but that wouldn't help us much.
A small point: Korea is thirteen hours ahead of the U.S. East Coast. Since the warning for April 10th was directed to embassies in Pyongyang, it seems reasonable to assume that they referred to Korean time, suggesting that whatever they had in mind could occur on April 9th in Washington, D.C.
Put my hand-crank radio, my generator electrical parts, and a few other nice-to-haves in a Faraday cage (and keep them there 'cause they should already be in there, dammit).