That file (dense.bmp) is one of built-in Distribution Maps by default located at C:\Program Files (x86)\Itoo Software\Forest Pack Pro\distmaps\images.
Actually, to prevent these problems, Forest Pack is double checking if all needed paths are listed there and offers you an automatic addition (see attached screenshot). Could be, you've skipped that step.
Anyway, it can be easily fixed by adding the path to these assets under the 3ds Max's configuration Customize - Configure User Paths - External Files stored in 3dsmax.ini. The ini file is located by default here:
C:\Documents and Settings\<username>\Local Settings\Application Data\Autodesk\3dsmax\<max version>\enu\3dsmax.ini
Hope that helps.