I had some trouble installing aspell in Emacs on my Mac, so here is what I did to make it work.
I installed Emacs 24.4 from the Emacs for OS X web site, and I installed cocoAspell from its web site.
Additionally I installed the German dictionary from the aspell dictionary web site accessible from the cocoAspell web site.
As described in the docs, I installed the additional dictionary directory into /Library/Application Support/cocoAspell/ by unzipping the archives from the cocoAspell site. This created a subdirectory for the dictionaries, in my case /Library/Application Support/cocoAspell/aspell6-de-2030222 in addition to /Library/Application Support/cocoAspell/aspell6-en-6.0-0 which was created by the cocoAspell installation itself.
Then, I created a new directory /Library/Application Support/cocoAspell/aspell6-combined and copied all files from the other two dictionaries directories into these new directories.
Then I edited the file /usr/local/etc/aspell.conf. I replaced the line
dict-dir /usr/local/lib/aspell-0.60
by
dict-dir /Library/Application Support/cocoAspell/aspell6-combined
All these action need admin privileges, so in most cases one has to use sudo to run the commands.
In the final step, I had to start Emacs, and navigate to Tools -> Spell Checking -> Customize and set the value for Ispell dictionary to default
After that, I could use the menu items on Edit -> Spell, and using