How you are installing it? According to Riak's documentation, you use:
sudo yum install gcc glibc-devel make ncurses-devel openssl-devel autoconf
And then, you build Erlang:
wget http://erlang.org/download/otp_src_R15B01.tar.gz
tar zxvf otp_src_R15B01.tar.gz
cd otp_src_R15B01
./configure && make && sudo make install