AbleCommerce Shopping Cart Software

Ok, I followed all the instructions on http://help.ablecommerce.com/ to set up PayPal Express Checkout. The pay pal express checkout button is showing up. However it is throwing an exception when I click on it. What am I missing? Any ideas?
Thanks a lot.

Here is the exception:
[SecurityException: That assembly does not allow partially trusted callers.]
CommerceBuilder.Payments.Providers.PayPal.PayPalProvider.SetExpressCheckout() +0
PayPalExpressCheckout.SetExpressCheckout() +26
PayPalExpressCheckout.Page_Load(Object sender, EventArgs e) +245
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +15
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +33
System.Web.UI.Control.OnLoad(EventArgs e) +99
System.Web.UI.Control.LoadRecursive() +47
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +7350
System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +213
System.Web.UI.Page.ProcessRequest() +86
System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +18
System.Web.UI.Page.ProcessRequest(HttpContext context) +49
ASP.paypalexpresscheckout_aspx.ProcessRequest(HttpContext context) +4
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +358
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64

ASP.NET can be configured for different trust levels. Your environment is most likely configured for "Medium" trust. This is a common setting for shared hosting environments. The API produced by PayPal for the express checkout feature requires full trust.

The only option I can offer at this time is to have your ASP.NET environment upgraded to Full trust.