Powershell custom activity

Hello, 

I'am trying to use the Powershell Activity in my script i do a simple write in a file but nothing happens and i have  this error 

System.InvalidOperationException: Une exception de type 'Microsoft.ResourceManagement.Workflow.WorkflowExtensionException' a été levée.
   à FimExtensions.FimActivityLibrary.PowerShellActivity.Execute(ActivityExecutionContext context)
   à System.Workflow.ComponentModel.ActivityExecutor`1.Execute(T activity, ActivityExecutionContext executionContext)
   à System.Workflow.ComponentModel.ActivityExecutor`1.Execute(Activity activity, ActivityExecutionContext executionContext)
   à System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime)
   à System.Workflow.Runtime.Scheduler.Run()

Any idea how to resolve this ? 

Thanks