I was trying to install some software/edit files with my main username(not root)
But I got this error [X] is not in the sudoers file. This incident will be reported.

I tried to edit the /etc/sudoers file by using sudo and su username but I got the same error!

How I'm supposed to add my username tosudoers file while I cant access it anyway?

I also tried to follow this guide here.

is the guide says, when booting 2 Operating systems(that's my case), then selecting Ubuntu, a menu will let you choose to start normally/open recovery mode/etc, but it's not happening for me, even if I hold shift no menu will pop up.

I installed Ubuntu by using Wubi and my Ubuntu version:12.04 LTS.