Running into the same issue. I have run the installer several times, also used "run as administrator" and entered (copy and paste) from my code records. Start P3D and Add-ons then FSUIPC. Not registered. 
Install File: 
Installer for FSUIPC4.DLL version 4.703 
Looking in registry for FSX install path: 
     HKEY_CURRENT_USER\SOFTWARE\Microsoft\Microsoft Games\Flight Simulator\10.0 
     Parameter"AppPath" 
Not there, so looking in: 
     HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft Games\Flight Simulator\10.0 
     Parameter"SetupPath" 
... NOT found! ... 
Looking in registry for ESP install path: 
     HKEY_CURRENT_USER\SOFTWARE\Microsoft\Microsoft ESP\1.0 
     Parameter"AppPath" 
Not there, so looking in: 
     HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft ESP\1.0 
     Parameter"SetupPath" 
... NOT found! ... 
Looking in registry for Prepar3D install path: 
     HKEY_CURRENT_USER\SOFTWARE\LockheedMartin\Prepar3D 
     Parameter"AppPath" 
... >>>  OK! FOUND Prepar3D!  <<< ... 
=========================================================== 
INSTALLATION FOR Prepar3D: 
AppPath="C:\Program Files (x86)\Lockheed Martin\Prepar3D\" 
Checking version of Prepar3D.EXE: 
... Version 1.1.2771.0  (Need at least 1.0.677.0) 
Checking compatibility with installed SimConnect: 
    Found SimConnect build 60905 (Original) 
    Found SimConnect build 195 (ESP Orig) 
    Found SimConnect build 61242 (SP1 May07) 
    Found SimConnect build 61259 (Acc/SP2 Oct07) 
Checking if there's already a version of FSUIPC4 installed in: 
       C:\Program Files (x86)\Lockheed Martin\Prepar3D\Modules\FSUIPC4.DLL 
... No previous valid version found. 
Prepar3D Modules folder created okay! 
Okay -- installed FSUIPC4 into "C:\Program Files (x86)\Lockheed Martin\Prepar3D\Modules\FSUIPC4.DLL" 
Looking for the current user's Application Data path:  
... found as "C:\Users\Boeing1\AppData\Roaming" 
Now finding \Lockheed Martin\Prepar3D\Prepar3D.CFG for all users, including this one 
Looking in "C:\Users\All Users\AppData\Roaming" 
 ... No Prepar3D.CFG there 
Looking in "C:\Users\Boeing1\AppData\Roaming" 
Found Prepar3D.CFG in "C:\Users\Boeing1\AppData\Roaming\Lockheed Martin\Prepar3D\Prepar3D.CFG" 
Now checking DLL.XML ... 
... There is a previous DLL.XML, checking for FSUIPC4 section. 
... FSUIPC4 section already exists but will be replaced. 
... FSUIPC4 section of DLL.XML written okay 
Now checking for a SimConnect.XML file ... 
... No SimConnect.XML file found. This is okay. 
Looking in "C:\Users\Default\AppData\Roaming" 
 ... No Prepar3D.CFG there 
Looking in "C:\Users\Default User\AppData\Roaming" 
 ... No Prepar3D.CFG there 
Looking in "C:\Users\Public\AppData\Roaming" 
 ... No Prepar3D.CFG there 
"Modules\FSUIPC Documents" folder created okay! 
Now installing additional files into the "Modules\FSUIPC Documents" folder: 
   Installed "FSUIPC4 User Guide.pdf" okay 
   Installed "FSUIPC4 for Advanced Users.pdf" okay 
   Installed "FSUIPC4 History.pdf" okay 
   Installed "List of FSX controls.pdf" okay 
   Installed "GlobalSign Root.exe" okay 
   Installed "FSUIPC Lua Library.pdf" okay 
   Installed "FSUIPC Lua Plug-Ins.pdf" okay 
   Installed "Lua License.pdf" okay 
   Installed "Lua Plugins for VRInsight Devices.pdf" okay 
   Installed "Example LUA plugins.zip" okay 
FSUIPC4.DLL installed and signature checked out okay! 
Deleted GlobalSign Root fix program ... no longer relevant 
FSUIPC shows up as unregistered 
Any suggestions, first time with Prepar3d and FSUIPC, never had any trouble before and have used it for years. Thanks for your continued support Pete! 
SC