Simply visit this page. Click the orange "Donate" button. Paypal will take it from there. Note that this is not tax-deductible. The government does not encourage its subjects to stand up for the blameless.
And yes, Julius was guilty as sin, but Ethel Rosenberg was guilty at most of minor misdemeanors. Her execution is a stain on America's honor over fifty years later.
Joel is only coincidentally named Rosenberg.