To add a repository you need to add the key for the repository then install the repository. For this example I will use the El Repo since it has the commands you need to run:
First you need to import the key using rpm --import:
rpm --import http://elrepo.org/RPM-GPG-KEY-elrepo.org
Then you run rpm with the -Uvh switch to isntall the repo (there are two installations depending on what version of CentOS/RHEL your using).
RHEL 5/CentOS 5
rpm -Uvh http://elrepo.org/elrepo-release-5-4.el5.elrepo.noarch.rpm
RHEL 6/CentOS 6
rpm -Uvh http://elrepo.org/elrepo-release-6-5.el6.elrepo.noarch.rpm
The error your getting is showing that particular package doesn't exist. If you found on the internet that the package should be available you might want to inquire what type of repository is required to download it. It seems you just want Python and MySQL to be available. I think that MySQL is available with the default repositories.
Install MySQL & PHP:
yum install mysql-server mysql php-mysql
yum install python26
For python26 you will need the EPEL repository:
The above links should be able to help you with getting the required repository for Python.
In general, you can also use RPM search engines to look for specific packages, download and install them using
rpm -ivh downloaded_package.rpm. Some of the better known ones are: