Quote from src/plugins/zandronum/zandronumbinaries.cpp

bool ZandronumClientExeFile::downloadTestingBinaries(const QDir &destination, QWidget *parent)

{
        #ifdef Q_OS_DARWIN
        // Can't do anything for Mac OS X at this time. :/
        return false;
        #else
// ...
}

It should probably instead be:

bool ZandronumClientExeFile::downloadTestingBinaries(const QDir &destination, QWidget *parent)

{
        #if !defined(Q_OS_WIN) || !defined(Q_OS_LINUX)
        // Can't do anything for other operating systems at this time. :/
        return false;
        #else
// ...
}

Because there are no OpenBSD binaries available from zandronum.com.