Step 1

Open up sign creator and set up the sign to the required size by double clicking on the white area

Step 2 :

Insert a text box in the filed you have just created,

Step 3:

Once you have a text box double click on it and select insert macro then XML as shown bellow.

Step 4 

In this image you can see all the necessary changes that you need to make to have an RSS feed.

That is the final step once you click OK you should have a RSS feed showing in your text box as shown bellow.

Another Option the setup above is that you can display multiple feeds in one sign and to do that just have to make a couple of small changes.

Change 1: Have multiple text boxes drawn instead of 1

Change 2: In every text box add a new macro (1 macro per text box)

Change 3: In Step 4 above  make the following changes 

                                                                 1. Un-tick Display Next item

                                                                 2. In first text box for Tag path select item(0) second text box follow same step but instead type item(1) and so on for all the separate text boxes

That would look as following