Copy and paste DIABDAT.MPQ directly into the DevilutionX folder.
When Diablo 1 was released in 1996, hard drive space was at a premium. To save space, the game's installer only placed the executable ( Diablo.exe ) and essential system DLLs onto the user's hard drive. The massive DIABDAT.MPQ remained on the CD-ROM. diablo 1 diabdatmpq
I can provide step-by-step installation instructions for your specific setup. Copy and paste DIABDAT