Is it showing the same security header is invalid error?
That error occurs for the following reasons:
Using Sandbox API Credentials through the live server
using Live API Credentials through the sandbox server
You must except the billing agreement with payapl after requesting the API credentials.
Make sure there are no leading or trailing spacers in the api credentials when you add then in the admin section.
sometimes the API credentials that paypal generates are just bad, try getting new api credentials.