Firefox Add-on RESTclient - How to input POST parameters?

I've installed Firefox RESTclient add-on but , I'm having hard time figuring out how to pass POST parameters. Is there a specific format to do this? Or is there any other tool which can be used to debug an REST API on Mac OS X ?

If you want to submit a POST request

1. You have to set the “request header” section of the Firefox plugin to have a “name” = “Content-Type” and “value” = “application/x-www-form-urlencoded”
2. Now, you are able to submit parameter like “name=mynamehere&title=TA” in the “request body” text area field

Here is a step by step guide (I think this should come pre-loaded with the add-on):

1. In the top menu of RESTClient -> Headers -> Custom Header
2. In the pop-up box, enter Name: Content-Type and Value: application/x-www-form-urlencoded
3. Check the "Save to favorite" box and click Okay.
   Now you will see a "Headers" section with your newly added data.

4. Then in the Body section, you can enter your data to post like:

   username=test&name=Firstname+Lastname
   

5. Whenever you want to make a post request, from the Headers main menu, select the Content-Type:application/x-www-form-urlencoded item that you added and it should work.