The instructions to install Riak from Basho’s wiki are a bit off for OS X Lion as you have to build it 32-bit for it to work. I’ve found the following to be effective at installing Riak 0.14.2 on OS X Lion:

$ curl -O http://downloads.basho.com/riak/riak-0.14/riak-0.14.2.tar.gz
$ tar xzvf riak-0.14.2.tar.gz
$ cd riak-0.14.2
$ CC=gcc-4.2 CFLAGS="-m32" LDFLAGS="-m32" make all rel
$ cd rel/riak
$ ulimit -n 1024
$ bin/riak start

At this point, you should have a running Riak installation. Have fun!