(command "_NETLOAD" "useArray.dll") returns NIL, it does not load.
DLL does not define a LISP function but is a pure C++ DLL.
I think dgorsman's vlax-invoke-method can be used.
But how.
dgorsman is correct, there is no way to load this dll unless it exposes a COM interface or it is created as an ObjectARX application that exposes a Lisp function interface. The NETLOAD command is used to load .Net assembly dll's, you can't load a native (C++) dll with NETLOAD.
If this was written by youself or you know the person perhaps you can ask them to write it in .Net, that is the easiest way forward IMO.
<added> actually, you would need to expose the .Net dll with a Lisp interface as well, to hook different runtime tecnnologies you need to provide some way for them to talk to each other and these interfaces are what make that possible.