Ben Alder is a complex mountain with a large plateau, clearly visible in the distance from the A9 when heading south. The approach is long, and may deter most, but in relatively dry conditions, most of it can be done by mountain bike, taking the sting out of a day trip. The lines described here are in the remote Garbh Choire, on the east face of the mountain. On a clear day, you should be able to see into the corrie from Dalwhinnie, which should help decision making as to whether to make the long journey or not.
Approach
By far the easiest and most logical approach is from Dalwhinnie, starting from the level crossing (make sure you do not obstruct the crossing when parking). Follow the well maintained track along by Loch Ericht, passing some grand looking buildings, before the gradual climb up to the start of the superb stalkers path at NN548787. From here, the path is cyclable, all the way up to the outflow of the Loch a Bhealaich Bheithe at NN512727 if you have the legs for it (and want a superb run down). Alternatively, it's reasonable to dump the bike at the 500m contour, beyond the now closed Culra Bothy (as of March 2014) just as the path starts to climb (NN515751). The path marked on the map on the east side of the loch is very boggy, but crossing the outflow of the loch may not be an option, so rounding the southern end of the loch to gain access to Garbh Choire may be your only option. As an indication, 3-3.5 hours should see you well into Garbh Choire from Dalwhinnie.
Other possibilities
"Right Gully" in Garbh Coire Beag may be worth a exploration. The current SMC guidebook for the area (2002) grades it as a III, but i've read elsewhere that it is only a Grade I, and that would agree with what I have seen of it. It's defined in it's top half, but then you would have to find a way through broken ground below. There are also a couple of nice looking lines into Coire na Lethchois, between the Short and Long Leachas. If anyone is interested, I can send pictures of these lines. There are many attractive looking shallow gully lines descending off Beinn Bheoil to the lochan as well. It's possible the north side of the mountain may yield something, but I haven't checked it out.
I've indicated that the Creag Meagaidh avalanche forecast is the nearest relevant forcast. Whereas this avalanche forecast does not actually cover Ben Alder, the mountains are relatively close to one another, and the cliffs have similar aspects.
Relevant Weather Forecasts
