As the warning says: "...will replace your current iCal data." All of your current data will be overwritten with the .icbu file.
I presume that you do not want to do that.
One method of accomplishing the integration without losing current data, is to use iCal>File>Export>Export... for each one of your current calendars. That method will create .ics files for the individual calendars. After creating those .ics files and saving them to a suitable location, you can then use the .icbu file to restore from the archive.
After the archive restoration is complete, you can go to the saved location and double click on the .ics files. You will be offered a choice to create a new calendar, or integrate into an existing calendar. Use the choice appropriate for your situation.