As of 5.24c the constants mod isn't usable. The mod is supposed to be accessible from the main menu, but isn't even when enabled. I re-downloaded the game to confirm this. I haven't seen any information on how to fix it here so here's what worked for me.
Open up the game client and go to the main menu. Then look at the black screen that opens alongside the game client. You should see this error message:
SCRIPT ERROR: GDScript::reload: Parse Error: Too few arguments for 'str' intrinsic function.
At: user://mods/Constants/constantsmod.gd:22
ERROR: reload: Method/Function Failed, returning: ERR_PARSE_ERROR
At: modules/gdscript/gdscript.cpp:580
SCRIPT ERROR: _ready: Invalid call. Nonexistent function 'new' in base 'GDScript'.
At: res://files/scripts/mainmenu.gd:125
If the text in the black screen matches the text above, this fix will work. Close the game then go to your mods folder and open up the folder for the constants mod (for me it was in C:\Users\AppData\Roaming\Strive\mods\Constants). You should see a file named 'constantsmod.gd'. Open the file, preferably with notepad++ as the code is easier to read with it than regular notepad. Look for the line marked 'var text = str()'. Its near the top, it will be line 22 if you opened it with notepad++. Change this line to :
var text = ''
That's two single quotation marks together, not a double quotation mark. Save the file and then open the game. The constants menu should now be visible at the main menu.