如何从MSI自定义操作调用的InstallScript调用DLL函数

I have a DLL named FSSetup.dll that I want to call from InstallScript code. And I want to call the InstallScript code from an MSI custom action. But there seems to be a problem in that I don't know where to find FSSetup.dll. The InstallScript function is starting and the whole install aborts when it gets to the line where it calls the DLL function. The MSI log reports: that I want to c