To create RSS feeds from your site just pass the right variables to external.php.
The values it is setup to take are:
category = type
city = city
state = state
country = country
ad owner = owner
minimum price = minprice
maximum price = maxprice
number of ads (default is 15) = limit
text search of title and description = searchtext
To utilize these you would call external.php in this manner:
external.php?type=1&city=dallas
Magpie is pretty much setup "right out of the box". It generally just requires a url. For the url construct as above the place the full url in the magpie spot -
http://www.yoursite.com/ads/external...ty&state=state
Do you have the developer or designer version? If you have the developer version I can help you change the query a bit for featured ads, newlistings etc.