You need MySQL, PostgreSQL and Riak up and running.
MySQL should be accepting TCP connections on localhost:3306.
PostgreSQL should be accepting TCP connections on localhost:5432.
Riak should be accepting TCP connections on localhost:8087.
MySQL and PostgreSQL should grant full access to user 'ejabberd_test' with
password 'ejabberd_test' on database 'ejabberd_test'.
Riak should be configured with leveldb as a database backend and -pz
should be pointed to the directory with ejabberd BEAM files.

Here is a quick setup example:

------------------
  PostgreSQL
------------------
$ psql template1
template1=# CREATE USER ejabberd_test WITH PASSWORD 'ejabberd_test';
template1=# CREATE DATABASE ejabberd_test;
template1=# GRANT ALL PRIVILEGES ON DATABASE ejabberd_test TO ejabberd_test;

-------------------
  MySQL
-------------------
$ mysql
mysql> CREATE USER 'ejabberd_test'@'localhost' IDENTIFIED BY 'ejabberd_test';
mysql> CREATE DATABASE ejabberd_test;
mysql> GRANT ALL ON ejabberd_test.* TO 'ejabberd_test'@'localhost';

-------------------
  Riak
-------------------
$ cat /etc/riak/vm.args
...
## Map/Reduce path
-pz /path/to/ejabberd/ebin
...

For version < 2.x:

$ cat /etc/riak/app.config:
...
 {riak_kv, [
            {storage_backend, riak_kv_eleveldb_backend},
...

For version >= 2.x:

$ cat /etc/riak/riak.conf:
...
storage_backend = leveldb
...