While it can be done in a config file, the way I do it is to right click on the Armory launch shortcut and select "properties"
edit the "Target" line with your custom path argument; example
"D:\Program Files (x86)\Armory\ArmoryQt.exe" --satoshi-datadir="E:\your path here" --datadir="D:\armory"
In this particular case, my Armory install is on the D: drive, yours may not be.
Replace "E:\your path here" with your Bitcoin Core data path (the folder with "blocks" and "chainstate" folders in it)
The --datadir arg is if you are using a nonstandard location for your Armory database. This folder will have "databases" folder in it, wallet files, and "ArmorySettings" and log files in it. Again, replace "D:\armory" in my example with your path.
When you think you have all the syntax right (check the spaces and such, we are talking command line here) be sure to click "Apply" in the bottom right of the shortcut properties dialog.
I hope this helps.
edit/
On rereading your last post, I wonder if you have properly pathed Bitcoin Core itself. The procedure is very similar. Edit the Core shortcuts target field.
https://bitzuma.com/posts/moving-the-bitcoin-core-data-directory/"D:\Program Files\Bitcoin\bitcoin-qt.exe" -datadir=E:\your path here
Of course correcting for the drive letters where you actually have your installs.
As always, upon making such changes, close Armory, check for and kill armorydb in task manager and delete the armory\databases folder before restarting Armory on top of a fully synced and running Core node.