To export your podcast subscriptions from iTunes:

1. In iTunes Click on "Podcasts"
2. Click File->Export
3. Set the file type to "Podcast Subscription Files (*.opml)" and click "Save"
4. On CastRoller click on, add podcast
5. Click the Browse and double click on your exported subscriptions
6. Click on the "Add Podcast(s)" button.

View this short video to see how it is done: