Installation of MusiCAD should be straightforward.
Complete the application-form at musicad.com/download
Read your e-mail with subject MusiCAD 4 application and click on one of the download links supplied in it.
Depending on the browser and computer configuration settings you will get some window asking for a destination suggestion (in fact it does not matter where you store your download, as long as you can find it again).
You can safely ignore any warnings regarding dangers of installing downloaded software; of course you intend to execute the downloaded MusiCAD software.
After downloading you start the installation program (something like mc4b835.exe) and answer the questions. Accept the default values supplied unless you understand the effect.
At the end of the installation the MusiCAD registration will start. Part of the registration is necessary for MusiCAD to function correctly; without providing a valid name and the dummy license nothing will work as expected... However, no information will be sent without your consent.
If you are running MusiCAD for the first time (or for the first time on a new computer) you should use 0 for a license; a license-key for another computer or name will definitely not work. Complete the registration form anyway.