Hi Peter, Windows 10 technical preview is not officially compatible with Parallels Desktop 9. The full list of supported guest OS is available at
http://download.parallels.com/desktop/v9/ga/docs/en_US/Parallels Desktop User's Guide/33321.htm
However I have just tested it in the labs and it seems to work fine. To install Windows 10 technical preview on Parallels Desktop 9 for Mac, Please follow the steps suggested below to install it.
1. Download Windows 10 from :
http://windows.microsoft.com/en-in/windows/preview-iso
2. Follow the steps suggested at kb.parallels.com/4729 to initiate the installation.
3. On the step 3, click locate manually.
4. Choose Image file and drag and drop the Windows 10 .iso and click continue,
5. Parallels wizard will prompt us to choose the OS,
6. Choose Other Windows > Other Windows OS.
7. On the next screen choose "Customize settings before installation" > Click continue.
8. Now Choose RAM 2 GB close the dialog box and click continue to install Windows and follow the onscreen prompts to complete the installation.
Click to expand...