Well, download it manually from http://www.opera.com and use dpkg -i
on downloaded package - that should install Opera.

Try: Applications>Add/Remove Opera is there.
Selecting INTERNET from the Left Menu makes it
easier to find....

Opera can be found in synaptic too. Or read this link

https://help.ubuntu.com/community/OperaBrowser

Ensure that your kernel is i386 or generic; there is no
version of Opera suitable for the amd kernel.

If you are trying to install the very latest build of Opera,
Build 698, ver 9.26, it may be better instead to look on
their site for the previous ver 9.25. When I upgraded
yesterday the downloaded .deb would not install via Aptitude
(it kept insisting that some other installer was running
concurrently) and I had to run dpkg -i in a terminal. It
installed ok but in the process wiped out
/boot/initrd.img-2.6.22-14-generic, and I had to rename the
.bak to get past grub. I too am a newbie, and the process
was Not Fun.

A good point on running a system that has a 64 kernel . Both in Linux and
Windows the kernel is buggy on running 64 bit. All I can say is run the 32 bit
kernel and you'll have less problems. Linux 64 bit is going to beat Windows.
Hands down. Yet, it is techo catching up what is new. if even haven dual crap on
the board. It does nothing if there is no support for it. As of now there is not
very good support for it.