Comcast does not really offer a way for customers to "unspam" an email address themselves. You can select the messages in the Spam folder and click the "Not Spam" button, but it will likely take a lot of clicking on a lot of messages to change this.
Their Security Assurance group might be able to fix this for you. Give them a call at 1-888-565-4329 (from http://constantguard.comcast.net/help/report-abuse/). Note that this is NOT one of the general customer service numbers. The first-line reps there are sometimes not very helpful. If so, be persistent, and keep at them until you get a satisfactory answer.
Without their help, the only thing you can do to prevent messages from going to the spam folder is to turn off spam filtering completely so that everything they deliver is placed in the Inbox.