Install:

Put the Kenjeran folder in:
                        _GTA IV/PC/DATA/MAPS

Copy and paste GTA.DAT & IMAGE.DAT in:
                                      _GTA IV/COMMON/DATA

Done
                                                                                                   

if you already got some other map installed you can copy these line in you GTA.DAT & IMAGE.DAT         
                                                                                                        
These line in GTA.DAT                                                                                   
                                                                                                        
# map files (IDE)                                                                                      
IDE platform:/DATA/MAPS/KENJERAN/KENJERAN.IDE                                                                
# map files (IPL)                                                                                       
IPL platform:/DATA/MAPS/KENJERAN/KENJERAN.IPL                                                                 
                                                                                                        
This line in IMAGE.DAT                                                                                  
                                                                                                        
#this has been ordered to reflect the order on the disk                                                 
platformimg:/data/maps/kenjeran/kenjeran	0                                                               
                                                                                                        