This is where you can either ask the user for a custom destination for your copy files, or you can obtain the destination from a reg key.
[Version]
Signature=$CHICAGO$
AdvancedINF=2.5,"You need a newer version of Advpack.dll"
[SourceDiskNames]
10="Title",,0
[SourceDisksFiles]
testfile.txt=10
[DefaultInstall]
CustomDestination=CustomDestinationSection
CopyFiles=CopyFilesSection
[CustomDestinationSection]
49001=DestA,1 ;<1=prompt user, 5=no prompt>
[DestA]
HKLM,"SOFTWARE\Test","InstLoc","Location Not Found in Reg. Use
default?","C:\default\folder"
[DestinationDirs]
CopyFilesSection=49001
[CopyFilesSection]
testfile.txt