Dear Gabriel,
If you are using the FAST v7 executable linked above, the name of the DLL is set within the executable. If you upgrade to FAST v8, the filename, including path, are set within the FAST input files. See the following forum topic for solutions to this problem: "DISCON.dll could not be loaded" - #2 by Bonnie.Jonkman.
The FAST v7 executable linked above does not consider foundation flexibility through a p-y model, so, a recompile of FAST v7 is required to get such functionality. This is discussed in the following forum topic: Turbine-soil interaction. Influence on the mode shapes., especially my post dated Feb 01, 2013, where I share an example UserTwrLd_DS.f90 file.
Best regards,