I don't use Mamp but normally with Apache there is a root directory called htodcs. If your mamp is configured to work with this as the root web folder then you should be able to add in a folder in here then set it up in DW to use this as the local site folder. Here are some links to docs that we have on setting up the testing server and the site in DW:
Defining a site in DW:
Installing a testing server with MAMP
Take a look at these to see if there is any more helpful information for your current setup. Based on the error you are getting it seems that there might be a problem with the site as it is setup in DW. It might be worth creating a whole new DW site to see if the problem is associated with that one site.