You should already have it on your ASA. There's no additional cost/licensing for it.
The ASDM is a separate file from the main firewall firmware software. If you do a dir of the flash, you should see the firmware file(s) as asa[version].bin and then a file for the ASDM as asdm-[version].bin. From the factory, the ASDM and the firmware should be matched up to work with each other. If you upgrade the firmware, you need to make sure the ASDM file you have on the ASA is compatible or if it requires upgrading.
You then need to make sure in the config the path is identified and called out for the ASDM file in flash memory. Next you need to enable the http server (it's ok as communication to the ASA is over SSL). And finally, you have to configure the allowed IP addresses or subnet to access the ASA via the ASDM. Unfortunately, I don't recall the exact commands at the moment.
The ASDM is a separate file from the main firewall firmware software. If you do a dir of the flash, you should see the firmware file(s) as asa[version].bin and then a file for the ASDM as asdm-[version].bin. From the factory, the ASDM and the firmware should be matched up to work with each other. If you upgrade the firmware, you need to make sure the ASDM file you have on the ASA is compatible or if it requires upgrading.
You then need to make sure in the config the path is identified and called out for the ASDM file in flash memory. Next you need to enable the http server (it's ok as communication to the ASA is over SSL). And finally, you have to configure the allowed IP addresses or subnet to access the ASA via the ASDM. Unfortunately, I don't recall the exact commands at the moment.