There isn’t a software package called `adobe-flashplugin` in your current software sources

I am trying to install the adobe-flashplugin on Ubuntu 14.04.
On Adobe's site I choose the "APT for Ubuntu 10.04+" option.
It opens Ubuntu software Center, but it tells me

There isn’t a software package called “adobe-flashplugin” in your current software sources

If you want to install Flash, you can install it using the command -

sudo apt-get install flashplugin-installer

Alternatively, to install Adobe flash from canonical partner repository -

Software & Updates -> Tab 'Other Software' -> Tick 'Canonical Partners'

sudo apt-get update

sudo apt-get install adobe-flashplugin

Its available in the partner repository.

If you goto System Settings > Software & Updates, then to the "Other Software" tab check the "Canonical Partners" checkbox.
You should get prompted for a password and then to reload.

After this you will be able to load adobe-flashplugin using the method in your question.

You can also install flash (exactly the same as both download flash from Adobe's website), using flashplugin-installer, also available from the software centre.