If you have a ForexTester 4 (or 3) license, download the software to the new PC, install it and activate with the same key.
Install it and go to Help → About Program → Click NOT REGISTERED, enter your name, email, registration key and activate the software.
If you have access to the old PC, you can check the key in Help → About Program. 
If you lost you registration information, please, contact us via email: support@forextester.com, and we will recover your registration key.
You can also transfer the data. Open C:\ForexTester4 on your old PC, copy the Projects, Templates and Data folders and replace them on your new PC. C:\ForexTester4 is a default location. If you installed the software into another folder, please replace the data in it.

If you own a Forex Tester 2 license, please do the following.
Download Forex Tester 2 to your new computer and install the software.

Run Forex Tester 2.

1. Go to Help → Register program menu.
2. Choose "I do not have a registration key" and press "Next".
3. Select "Yes, I already paid" and click "Next".
4. Fill in the key request form in the next window. Choose the option "Old key does not work" as your reason for requesting the registration key. If you forgot some contact information, you can write “Don’t remember”.
5. Press the "Request key" button.
6. We will generate the registration key for you and send it via email within the next 24 hours (usually much faster).

If you have access to the old computer, you can move the data to a new PC. Please open the C:\ForexTester2 folder on your old PC. Copy the Projects, Templates and Data folders to USB flash drive. Then replace these 3 folders on the new PC.