It's been a while, so I can only provide some general help.
You'll need to open Items, go to entry 241(column name should have value the_cube) and uncheck SubsciberOnly in bitmask02.
Then open Quest and go to entry 245. Uncheck the subscriberOnly column, then change offerReward to 208
Then open Treasure go to entry 458(questcubereward) and uncheck subscriberOnly.
The open Inventory, go to entry 55(column: description, value: cube), and delete the text in both playerPutRestricted and playerTakeRestricted, don't replace it with 0, 1, or -1 just delete it.
After modifying a file, you'll need to save it(File -> Save, not Save As), I guess you could export it instead.
You will need the correct folder structure(if you don't already), so for editing the quest file:
data/excel/quest.cooked.txt(or quest.txt if you exported it)
Then you run hellpack.
I have my modding folder structure like this:
Reanimator/data/excel
Within the Reanimator folder is the Reanimator and hellpack programs(and the other files).
Once you've run hellpack, go into Reanimator/data and copy sp_hellgate_1337.idx and sp_hellgate_1337.dat into your Hellgate london/data folder, then either run Reanimator(to check that the changes were made), or run the game. Running reanimator I find to be quicker to check if the changes actually changed.
When you run the game, then you should receive an on screen message saying that Nemo has a surprise for you, or something along those lines. And you should be able to interact with him and get the cube from him.
I think that was all that had to be completed to get the cube.
I know the exception message is saying that it can't find part of the specified path, but I don't know why it would say that.