If you haven't already, create a new Arc2Earth Account to get a Free Arc2Earth Lite serial number. If you have an existing Arc2Earth Serial Number, make sure to enter it in signup form to connect your existing account information.
From the Account page, click on the Download Installer button. If you are the account admin for your company, you can distribute the installer as needed. There is currently no public link for the new installer.
After the install completes, the A2ELicense.exe will be run to complete the activation process. If you do not see this window after the install (or if it includes items with errors), you will not be able to use Arc2Earth or Arc2Earth Sync. Enter your Serial Number in the window when prompted, you can change your Serial Number in the futrue using the Help menu on the main Arc2Earth Toolbar in ArcMap.
The majority of installation issues arise from lack of Administrator rights or Internet connection problems.