System.ComponentModel.Win32Exception when executing F# Nunit unit test code with Mono

I'm trying to use F# unittest on Mono. I use Mac OS X. I have the following environments

nunitFramework=.../bin/mono/NUnit.2.6.4/nunit.framework.dll
console=.../bin/mono/NUnit.2.6.4/nunit-console.exe
fsUnit=.../bin/mono/NUnit.2.6.4/FsUnit.NUnit.dll

This is F# code:

namespace HelloWorld.Core
module Hello = let SayHello name = "Hello"

This is the unittest for it.

module HelloWorld.Tests.Hello  
open HelloWorld.Core.Hello
open NUnit.Framework
open FsUnit

[<Test>]
let shouldSayHello () = Assert.AreEqual("Hello World!", SayHello "World")

I compile the code and unittest:

fsharpc --target:library HelloWorld.fs
fsharpc --target:library -r:HelloWorld.dll -r:$nunitFramework -r:$fsUnit HelloWorldTest.fs

I run the test: mono $console HelloWorldTest.dll to get the error messages.

ProcessModel: Default    DomainUsage: Single
Execution Runtime: mono-3.5
Unhandled Exception:
System.ComponentModel.Win32Exception: ApplicationName='mono', CommandLine='--runtime=v4.0.30319 "/Users/smcho/Dropbox/smcho/bin/mono/NUnit.2.6.4/nunit-agent.exe" cc695a32-96df-4346-bfda-e5547d7acc87 tcp://127.0.0.1:58755/TestAgency', CurrentDirectory='', Native error= Cannot find the specified file
  at System.Diagnostics.Process.Start_noshell (System.Diagnostics.ProcessStartInfo startInfo, System.Diagnostics.Process process) [0x00000] in <filename unknown>:0 
  at System.Diagnostics.Process.Start_common (System.Diagnostics.ProcessStartInfo startInfo, System.Diagnostics.Process process) [0x00000] in <filename unknown>:0 
  at System.Diagnostics.Process.Start () [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) System.Diagnostics.Process:Start ()
  at NUnit.Util.TestAgency.LaunchAgentProcess (NUnit.Core.RuntimeFramework targetRuntime) [0x00000] in <filename unknown>:0 
  at NUnit.Util.TestAgency.CreateRemoteAgent (NUnit.Core.RuntimeFramework framework, Int32 waitTime) [0x00000] in <filename unknown>:0 
  at NUnit.Util.TestAgency.GetAgent (NUnit.Core.RuntimeFramework framework, Int32 waitTime) [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) NUnit.Util.TestAgency:GetAgent (NUnit.Core.RuntimeFramework,int)
  at NUnit.Util.ProcessRunner.Load (NUnit.Core.TestPackage package) [0x00000] in <filename unknown>:0 
  at NUnit.ConsoleRunner.ConsoleUi.Execute (NUnit.ConsoleRunner.ConsoleOptions options) [0x00000] in <filename unknown>:0 
  at NUnit.ConsoleRunner.Runner.Main (System.String[] args) [0x00000] in <filename unknown>:0 

What might be wrong?