Unable to turn on seamless mode in VirtualBox

I'm not able to switch to Seamless Mode in VirtualBox with Ubuntu 10.04 or 9.10 as guest OS's. I have tried the seamless mode with XP as guest OS, and it works absolutely fine. The host OS is Windows 7.

I have in-built Via/S3G Unichrome Pro IGP graphics, and I have allocated 30 MB out of 64 MB of graphics memory. I've also 1.5 GB of RAM.

I also tried installing Guest Additions but it still didn't work. VirtualBox version is 4.0.4r70112. The Host+L does not lead to seamless mode. i.e. under the Machine menu in Virtual box window "Seamless mode" is disabled (comes in grey color). How can I get the seamless mode to work? Please help!

For seamless mode (Host+L) Virtual Box Guest Additions of the same program version need to be installed in the guest OS.

How to install Guest Additions see also this answer. In some cases you will simply need to reinstall the Guest Additions if they don't run the way they should.

Turn off Nested Paging in the System tab and restart the guest.