What do you mean by activate external memory? To have any memory other than the memory that comes with the device, would require a SD card for it. Which you would have to purchase and install on the tablet. Make sure the card you buy actually works with your tablet, as ones with to much space or the wrong format just won't work.
If you already have a card and inserted it into the tablet, then it should work just fine. Unless you are getting some kind of error message.
Now, if you are actually try to to turn the SD card into "Internal" storage rather than "External" storage, that is a whole different animal. You would first have to be running a device that is using Android "Marshmallow" (a.k.a. 6.0) and the manufacturer would have to have not restricted the option to make that change on the the tablet. They do restrict them often. If you have the right version of Android, you can try the following...
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.