If you have a CD just insert it into your computer. Under Windows the Installation Choices dialog should automatically open:
If this dialog does not open double click on the program file setup.exe contained on the CD.
From the Installation Choices dialog select the program you want to install. If you have a registration code, click on Install a Registered Vensim Application and then enter the registration code to start the installer.
On the Macintosh just open the CD folder and look for the name of the installer you want. You should install the program first, then the models, and then the help file.
If you downloaded the Vensim installation program double click on the first file (for example, vendss32.exe for Vensim DSS) you downloaded. This will be in the directory you chose when your web browser queried you for the location to save to. (It is important to remember where you saved it to).
If you are on the Macintosh and the download files are still binhex (.hqx) files you will need to convert this to an application. There are a number of programs that do this.