how do i access pound sterling sign on USA purchased MBA?

Do you live in the UK?

opt-3, opt-shift-2

Or, switch to the British input source and it is shift-3 and opt-2.

To change Input Source, open Keyboard System prefs, Input Sources, click Add (–) button, and select English>British.

I'm not running Yosemite on the system I'm using at the moment and don't have time to reboot, but...

Option key + 3 key provides the £ symbol on the US keyboard layout.

Shift key + Option key + 2 key provides the € symbol on the US keyboard layout.

, System Preferences, Language & Text, Input Sources should allow you to checkmark turn on Keyboard & Character Viewer, and checkmark Show Input in Menu Bar. That puts a keyboard icon on the top menu bar. Then selecting the icon permits you to select Show Keyboard Viewer and that dynamically displays a miniature keyboard onscreen showing all keys and the alternate characters displayed when one or more 'helper keys (Shift, Function, Control, Option, and Command) are pressed.

Option-2 for the US Keyboard

Shift-3 for the UK Keyboard

Keyboard selection can be changed in the menu after adding it in System Preferences: