If you're trying to use the Munchkin API but not having any success, try these tips:
In the Munchkin Setup page, check the box that says Enable Munchkin API, put in a private key (which can be any text), and click Save.
Check and make sure that any quotation marks or other characters that require escaping are properly escaped. See this document for more.
Check to make sure you have only a path listed in the url parameter -- no ", no domain, and no URL parameters.
This could be caused by a few issues:
Include an email address
Email address is required when using the associate Lead call. If you don't use this, the call will always fail.
Marketo expects the labels (like "Email" and "FirstName") in a specific format. You can get these labels in the Field Management page of the Admin section. When you get there, click the Export Fields button. The spreadsheet will have the API names for your fields.
Check the hash value
The hash value is the SHA1 hash (non-HMAC version) of your API Private Key (not your Munchkin ID) concatenated with the lead's email address, as in firstname.lastname@example.org. The hash value must be lowercase.