install ArmA 2 first and then when you install Operation Arrowhead it will detect ArmA 2 and install OA to the same folder.
after both are installed then apply the 1.60 patch. it will patch both ArmA 2 and Operation Arrowhead.
1.60 patch: http://www.armaholic.com/page.php?id=15831
you are familiar with mod folders from using them with OFP?
the I44 download is a .zip file that contains several folders with the contents of the "mod".
extract the contents of the downloaded .zip file to your ArmA 2 folder.