Hi,
mojoPortal can currently use Authorize.NET, PlugNPay, PayPalDirect, PayPal Standard, and Google Checkout. In order to use any of them you have to configure them as documented here.
storedemo.mojoportal.com uses Authorize.NET with a test account processing payments through their test server. I cannoot demo PayPal on the demo site because using the PayPal sandbox only works for me when logged into my own PayPal sandbox account, there is no way to make a public demo processing through the PayPal sandbox. Similarly with google checkout, to try it users would have to have a google sandbox account and I would have to have an SSL certificate on storedemo.mojoportal.com for it to work.
On this site, mojoportal.com, the store uses only PayPal and Google Checkout because those are the low cost payment gateways. I cannot yet afford to use Authorize.NET on production. So I have this site configured with my production PayPal and Google Checkout accounts and I have an SSL certificate which is required for google to post orders to the site.
More info about our current WebStore feature is here: http://www.mojoportal.com/e-commercesolution.aspx
It is not designed to work with your .aspx pages, its designed to do what it does and its a work in progress.
Hope it helps,
Joe