These instructions can be used for R2014a and later. Earlier releases use a different installer.
Step 2: on the left side, select "Show More"
Step 3: still on the left side, select Show All
Step 4: click on R2014a (or as appropriate for your needs)
In the middle, a blue box will offer a platform.
Step 5a: If it is the wrong platform for you, click on the downwards triangle
Step 5b: and then click on the platform you do want.
Steps 5a and 5b may be omitted if it is already showing the platform you want.
Step 6, when it is showing the platform you want, click the blue button
Step 7: the installer will start downloading automatically, or possibly you will be asked where you want to store the downloader. The message that comes up will also show what MATLAB Central username you will need to authenticate against for this license.