Hi here is a way to install it.
Open up a terminal and type
and hit enter, it should ask you for your password. So type it in (it wont show the password by the way). Tell me if you can't get past this stage.
Then it should ask you for a new password so type one in (this won't change your normal login password).
Then confirm it and hit return.
Type the new password in and press enter.
Press enter if it asks you anything.
dpkg -i opera (don't press enter here, instead press tab and then enter)
Then just do CTRL-D twice and terminal should close. Opera should now be installed