As long as you don't want to modify the insides of ASLUND itself, you can put all your user analysis code, and histogram/ntuple i/o in the routines in ~/Aslund/Aslund/user. Specifically, book histograms in uinit.F, do analysis and histogram filling in uanal.F and do histogram printing/output in udone.F.
Other control can all be done in the files in workdir and workdir/Aslund. In particlular, you can change the number of events to analyse (default=10) using the card NEVT in the file runtime.input.
By default, the program generates decays of B0 to pi+pi- on one side of the event, and inclsive B decays on the other. If you want to change this, you need to modify the defaults in the generator interface BEGET (BaBar note 149, which can be found on the Web). This can be done using the cards at the bottom of the runtime.init file. For example, if you want to change the final states for B0 decays, one way is to provide your own replacement decay table. You will need to know about jetset decay tables to do this. Having created your decay table, put it in a file called eg. workdir/Aslund/channel.newdat and refer to it using the JETD card at the bottom of runtime.input.