Select to install it in P3Dv4, but when it gives you a blank box of where to install it (Please select the location where Prepar3D is installed), browse to the install location for P3Dv5, and click on install. Once that is done, you might need to add some lines to the P3Dv5 dll.xml file.

Open the Run Command (press the Windows Key + R to open it), then enter this: %AppData%\Lockheed Martin\Prepar3D v5. This will open the Prepar3D v5 folder, and you will then need to search for the dll.xml. Once found, Right Mouse click on it and select Open with and select Notepad (you might need to Choose another App to be able to select Notepad). Once it's open in Notepad, add the following:

  <Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
  </Launch.Addon>

at the bottom, but above the final line </SimBase.Document>, like this:

  <Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
  </Launch.Addon>
</SimBase.Document>

Note. If you have any other Captain Sim product(s) installed in P3Dv5, then the part about adding the info to the dll.xml file won't be needed as it will already be there.

Finally, you will need to copy the three 777-200 Cleared for takeoff (KSEA) (.cs.config, .FXML, and .wx) files from the Documents\Prepar3D v4 Files folder, to the Documents\Prepar3D v5 Files folder.

I'm pretty sure that's all you will need to do to get the 777 II Captain working in P3Dv5. It's working fine for me.