I have downloaded rpm from mysql website.
Those packages are considered examples. (For the future: Nobody uses 5.5.15 yet.)
And there are no or few extensions available.
Also : You already have a stable version installed : 5.1.56.
'rpm -ivh' is an "exception command", used if you want 2 libraries with
the same name installed at the same time. Or if you are absolutely sure,
that no package with <name> is installed.
The install command used by yum, etc. is 'rpm -Uvh'.