I've used Arvixe for a year now.
Login to your Arvixe Control Panel
Hover over the big Web(world) image and select Websites from the menu that appears
Next, In the right panel under Web Sites the name of your Website should be there.
It's a link. Click on your website name
On the next page there are a dozen tab pages you can select from, click the Extensions tab
Now in the left column next to ASP.NET: select (4.0) Integrated in the drop down list.
Then in the lower left corner of the page click the "Update" button.
Wait about 30 seconds till the page finishes.
OK... Now your new Website is ready to use mojoPortal with Dot.Net 4.0