How to integrate PayPal express checkout?

I'm developing a wordpress plugin that needed the PayPal Express checkout feature.

I was following this article from paypal site on how to Integrating the In-Context Checkout Feature (http://ift.tt/1NcGme4) which gave me the following form on 1 step.

<form method=post action=http://ift.tt/1HfUUmN;
    <input type=hidden name=USER value=API_username>
    <input type=hidden name=PWD value=API_password>
    <input type=hidden name=SIGNATURE value=API_signature>
    <input type=hidden name=VERSION value=XX.0>
    <input type=hidden name=PAYMENTREQUEST_0_PAYMENTACTION value=Sale>
    <input name=PAYMENTREQUEST_0_AMT value=19.95>
    <input type=hidden name=RETURNURL value=http://ift.tt/1NcGlqn;
    <input type=hidden name=CANCELURL value=http://ift.tt/1HfUVHi;
    <input type=submit name=METHOD value=SetExpressCheckout>
</form>

I have added my own API_username, API_password & API_Signature from PayPal but when submitted it shows ACK=Failure.

Following is the message from paypal I'm geting:

TIMESTAMP=2015%2d10%2d30T05%3a27%3a09Z&CORRELATIONID=24cb45b8dd36b&ACK=Failure&VERSION=0%2e000000&BUILD=18308778&L_ERRORCODE0=10002&L_SHORTMESSAGE0=Security%20error&L_LONGMESSAGE0=Security%20header%20is%20not%20valid&L_SEVERITYCODE0=Error

Because of this I cannot go to step 2 on the PayPal documentation, so need help here. What am I doing wrong?

via Chebli Mohamed