Sorry but I can't seem to identify the folder where the Ascii Importer stores format files. My edited format file does not appear in the Importer file dropdown. I have tried pasting the file into all AmiBroker folders that contain format files but that didn't work.
Under the Amibroker main folder, there is a "Formats" folder. drive:\path\Amibroker\Formats
You can place your .format file there, eg. custom1.format
Then look for the special file called import.types
Default ASCII (*.*)|*.*|default.format
It has the following format that you specify so that your format appears in the dropdown and you can also specify a pattern to filter those specific data files you want.
This filters all csv files beginning with nsedata like nsedata210701.csv
The actual import definition is stored in the NSEdaily.format file in the Formats folder.
For creation of format file you do not even need to lookup folders.
You just have to check "Add current settings to ASCII importer defintions" of Import wizard as seen in the KB article of first link.