Cannot post data to a plugin controller action

I have a form in one of my templates

 <form method="post" action="" accept-charset="UTF-8">
    <input type="hidden" name="action" value="paypalPayments/payment/pay">

     [...] 
 </form>

I also created a simple plugin like so:

//PaypalPaymentsPlugin.php
 <?php 
 namespace Craft;
 class PaypalPaymentsPlugin extends BasePlugin {

    function getName() {
       return Craft::t('Paypal Payments');
     }

     function getVersion() {
       return '1.0';
     }

     function getDeveloper() {
       return 'Paul Timoce';
     }

     function getDeveloperUrl() {
       return "http://paultimoce.net";
     }
 }

And also a controller file inside a controller folder:

 //File PaypalPayments_PaymentController.php
 <?php 

   namespace Craft;

   class PaypalPayments_PaymentController extends BaseController {

     public function actionPayWithPaypal() 
     {
        echo "I'm in...";
     }
   }

All I want to do is post some variables through my form and process them with my plugin controller but every time I submit my form I hit the 404 page...

Do you guys have any idea as to why this is happening? I also installed and activated the sample cocktailRecipes plugin trying to do the same thing but unsuccessfully also... I tried to point my browser to /actions/paypalPayments/payment/pay and /actions/cocktailRecipes/ingredients/saveIngredient but also no luck... I always get redirected to the 404 page.

Can you help?