Magento: Get the add to cart url of a product

First, create the product, then pass this product object to the checkout/cart helper to get the add to cart url.

$product=Mage::getModel('catalog/product')->load($productId);//load the product by product id
$product=Mage::getModel('catalog/product')->loadByAttribute('sku',$skuNum);//or load the product by sku number
$product=Mage::getModel('catalog/product')->setStoreId($storeId)->loadByAttribute('sku',$skuNum);//or load the product from a given store id

echo Mage::helper('checkout/cart')->getAddUrl($product);//Get the add to cart url

If you are in a block which extends Mage_Catalog_Block_Product_Abstract, you can use this line of code in the block class itself or in the phtml template file of this block to get the add to cart url.

$this->getAddToCartUrl($product);