Currently the ability to display the Manufacturer Logo URL and Manufacturer on the product list design isn't working correctly. The issue has been logged with our QA team and hope to have it working again in the future.
Once we resolve the issue, you should be able to add the following code to your custom product list design.
CODE
<img src="<%Product.ManufacturerLogoUrl%>" alt="unavailable" border="0" />
Since the feature doesn't work currently, I can not test the code. Once the issue has been resolved I will test the code and update this post.