I also have problems with logging and here is what I did to get log file.
I make FastMM5.INC in FastMM5 directory.
{$define FastMM_EnableMemoryLeakReporting}
{$define FastMM_FullDebugMode}
{$define FastMM_UseOutputDebugString}
{$define FastMM_ClearLogFileOnStartup}
{$define FastMM_EnterDebugMode}
Then, included FastMM5.INC in FastMM5.pas before interface.
unit FastMM5;
{$Include FastMM5.inc}
interface
Now, when I add FastMM5 in some project I'm getting log file.