Use F# Snippets instead of Pastebin :-). If you post an F# snippet there, it will add colorization and tooltips (just like those you see in F# in Visual Studio) and you can load the snippet to Try F# too.
For example, this sample snippet has a "tryfsharp.org" link that takes you to Try F# and loads the snippet using the URL: http://www.tryfsharp.org/Create?codeUrl=http://fssnip.net/raw/iF
If you wanted to use Pastebin, then Pastebin needs to have clientaccesspolicy.xml
file in the root (which tells Silverlight that accessing that domain is allowed). You can have a look at the file on fssnip.net So, this is not a feature request for Try F#, but for Pastebin.