Step 1: First make sure that you are logged in as an Administrator.  Then move to the installation folder (C:\Program Files\OneSafe Outlook PST Repair ) of software.
Step 2: Find the PhoenixDLL.dll in the folder
Step 3: Click on ‘Start Button’ and open ‘Run’ Window.
Step 4: Type regsvr32 command in the Run Window
Step 5: Now again open installation folder of ‘OneSafe Outlook PST Repair’ and select PhoenixDLL.dll file. After selecting this file, simply drag the file and drop into Run Windows as displayed in the image given below.
Step 6: After dropping PhoenixDLL.dll file, the path will be displayed - regsvr32 "C:\Program Files\OneSafe Outlook PST Repair\PhoenixDll.dll".

Step 7: Now click on ‘OK’ button. You will receive a pop-up on screen with this message - “DIIRegisterServer in C:\Program Files\OneSafe Outlook PST Repair\PhoenixDll.dll succeeded”.

Note: If you are using 64-bit Outlook, then registration of PhoenixDll.dll will be completed through similar process as mentioned above.

Source:Stellar Phoenix KB