To be able to install Ubuntu Netbook Remix in Virtual Box, you need to convert the .img file to Virtual Box .vdi file. To do so, enter the following command in terminal VBoxManage convertdd ubuntu-10.04-netbook-remix-i386.img ubuntu-10.04-netbook-remix.vdi .

At first, i encountered FATAL: No bootable medium found! System halted. This was solve by the following setting.

IDE Primary Slave: ubuntu-10.04-netbook-remix.vdi
IDE Primary Master: Netbook.vdi

To have this configuration, power off your virtual netbook, go to settings and click storage. Click the small [ + ] icon to add virtual storage media. Use the setting above. Reboot and now you have it.