Hi .. you can check your code against this code .. this has the continue shopping, perhaps you might be able to spot what you might be missing in your code ..
<form target="paypal" action="
https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="image" src="
http://www.xxxxxx.com/xxx.jpg" border="0" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<img alt="" border="0" src="
https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
<input type="hidden" name="add" value="1">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="business" value="
emailaddress">
<input type="hidden" name="item_name" value="product">
<input type="hidden" name="item_number" value="xxx">
<input type="hidden" name="amount" value="price">
<input type="hidden" name="no_shipping" value="0">
<input type="hidden" name="return" value="
http://www.xxxxxxxx.com/thankyou.html">
<input type="hidden" name="cancel_return" value="
http://www.xxxxxxxx.com/contactus.html">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="bn" value="PP-ShopCartBF">
</form>
You can take this code, modify things to apply to yourself (I am sure you can see where) and try it ..