Yes, you must aerate. Possibly, if you racked onto the entire trub of a previous batch, you would not have to. But you would probably end up with off flavors. I don't know how you fix a lack of oxygen at pitching at this point, sorry.
How long has it been fermenting? If it is actually stalled, you will probably do more harm than good by adding oxygen. I also doubt your gravity will drop more soley based on pitching more yeast. The yeast that is already in there should still be viable.
If you are fermenting in a controlled environment, I would recommend you raise the temp to 68-70 if you are below that, and maybe rousing the yeast a bit without getting too crazy (swirl the carboy and try to re-suspend the yeast).
I wouldn't aerate for fear of oxidation. I would make a starter with the nottingham and some DME and wait till krausen appears before pitching though. Even though starters are not normally recommended for dried yeasts, in the case of a stalled ferment it's better to get the yeast firing first.