A stock I bought about 3 years ago is now worthless. I can't sell, it has no value. How can I claim this loss? Can I simply use Schedule D? Or is there something else I have to do?

I see it was over one year since this question was asked, so I assume you found an answer already.

Before you can use this tax break, the stock must be totally worthless. You should list your stock as being liquidated on the last calendar day of the year. Don't bother paying a broker to "sell" for zero dollars. Make sure you enter the correct purchase date and cost basis, and use the correct part of Schedule D so that it is treated as the appropriate long- or short-term capital loss.

If the stock was held in an IRA, you cannot deduct the loss.