To build SAP Add On for 64bit Computers, there are several way to do such as AddOnRegDataGen, Visual Studio....In this, I am going to show with Visual Studio way. You can follow the step as below
I. Part 1
1. If it is the project, you need to create new project
2. If it is the old project mean it has source code already, so you need to clean your project ( delete all file and folder in Bin Folder )
3.Add UI and DIAPI to your add on project reference from
C:\Program Files\SAP\SAP Business One DI API\DI API 90\SAPbobsCOM90.dll and
C:\Program Files\SAP\SAP Business One\SAPbouiCOM.exe
4. On project propertied, Build: keep plateform and target as "Any CPU" and Build your project
II. Part 2
1. Create your installer project with B1DE and choose " EXE " from Part1 Step4
2. Click on your Installer Solution and Press Ctrl+H( Find and Replace) and Type " AddOnInstallAPI.dll " by replace to "AddOnInstallAPI_x64.dll"
3. Go to project propertied=>Compile=>Target CPU is x64 and Build you project
4. R-Click on AddOnRegDataGenFile\AddOnRegDataGen.bat
5. Open your .ard file with Notedpad and replace << platform="N" >> to << "platform="X">> and save.
After you followed the steps below, you can try to register your .ard file to Add On Adminstrator in SAP Business One.