if you can connect through your N900 then access the applications manager (menu > more>applications manager) then click download, enable extras-testing, (search 'N900 repositories') then click 'All' and type 'U' to skip through to the applications that start with a 'U' and you'll hopefully find 'USSD widget'. download and install, then access your desktop's settings by tapping on an empty spot, then tapping on the cog at the top left corner, then choose add widget...USSD widget.
tap at the application's settings button...then just type at the application what you would normally enter at a dial pad. exit and tap every time you want to refresh.
actually if you got it, you may realize that it is easier than the traditional method we're used to, thanks to the USSD widget author.
