Are you trying to update the nunit console that ships with Mono? You cannot update this simply using nuget.
NUnit, nuget, Mono. :
nuget install NUnit.ConsoleRunner
NUnit :
NUnit.ConsoleRunner.3.7.0/tools/nunit3-console.exe
nunit3-console.exe :
mono full/path/to/NUnit.ConsoleRunner.3.7.0/tools/nunit3-console.exe
, nunit3-console, nunit script /Library/Frameworks/Mono.framework/Versions/Current/Commands/nunit-console
#!/bin/sh
exec /Library/Frameworks/Mono.framework/Versions/5.8.0/bin/mono --debug $MONO_OPTIONS /Library/Frameworks/Mono.framework/Versions/5.8.0/lib/mono/4.5/nunit-console.exe "$@"
script, NUnit, .
script /Library/Frameworks/Mono.framework/Versions/Current/Commands/
, Mono.