If you have already paid for the product, then go near the top of this page and click on "My Community Profile". On the left side of the resulting page, most of the way down, there is a link to download your software products.
This requires that a license for that version is associated with your MATLAB Central login.
If you are admin'ing for a site then you need to get yourself listed as a technical contact for the license or as a download contact in order to download previous versions.
The "Get Trial Software" that IA mentioned is for the current release only, not past releases, unless perhaps you are under SMS (Software Maintenance Service). It is, though, the only mechanism for people without existing licenses to download the software.
For anything not covered here, you should contact Mathworks Sales or Mathworks Technical Support
