I'm not sure if you are talking about liquid or dry malt extract, so I'll talk about liquid malt extract first.
I usually don't use leftover liquid malt extract-- it usually won't work with the next beer I want to brew, and I just don't feel like obsessing over storage of another beer ingredient. If I were
to store liquid malt extract, then I'd store it in the original container to avoid the hassle of transferring it, store it in the refrigerator, and, if possible, remove as much air as I could from the storage vessel. If you're going to use the leftovers soon, though, you could just not worry about removing the air.
Another option is to use dry malt extract, which stores much better than liquid. You don't need to refrigerate it, and as long as you keep it sealed up and dry, you should have no worries about long term storage. If you want to convert a recipe that calls for liquid malt extract (LME) to dry malt extract (DME), then use 20% less DME since it is more concentrated than LME.
As for the hops, AAU is an acronym for alpha acid units, which is the product of the weight of the hops in ounces and the percentage of alpha acids (relates to amount of bitterness) in that particular batch of hops. For example, your recipe calls for 5 AAU of Cascade. You need to find the combination of weight and alpha acid percentage that, when multiplied, equals 5. If your Cascade hops are rated at 8.4% alpha acid, then you would need 0.6 ounces to get to 5 AAU (8.4% x 0.6 oz. = 5 AAU). This way of specifying hops allows for changes in alpha acid percentages in crops from year to year.
Here's a longer explanation by someone who knows more about brewing than I do: How to Brew - By John Palmer - Hop Measurement
The Hallertau hops are probably not given in AAU because they are meant to be used at or near the end of the boil and will contribute very little to the bitterness of the beer-- they're meant for adding flavor and aroma.