To work with PayPal digital goods you need to use PayPal Express Checkout system and then get the API credentials from the PayPal site.
Use the following link to sign up with PayPal :
Express Checkout sign up
You select the Express Checkout option and then either create a new account or convert your existing account into a Business account.
I have had some problems in converting an existing account to a Business account that will work with express checkout and I have found that the best way is to create a new PayPal account from scratch using the above link.
This sets up your account to work with Express Checkout.
Now you can get your signature details as follows :
Log into you PayPal account.
Click on the ‘Profile‘ tab.
Click on ‘My Selling Preferences‘ on the left hand side of the page.
For API Access, click on the ‘Update‘ link.
This takes you to the ‘API Access‘ page.
Click on ‘Request API credentials’ link and this takes you to the ‘Request API Credentials‘.
You need to select the ‘Request API signature’ so that you can obtain the three components of :
API Username
API password
API signature
Make sure that the Request API signature radio button is selected.
Then click on ‘Agree and Submit‘ button.
Copy all the details and store in a secure place on your PC.