Missing file error although file exists

Hi,

I'm working through SQL Server 2008 Reporting Services Step By Step. When I tried to run a report during one of the exercises I got the below error:

An error occurred during local report processing.
Could not find file 'C:\Users\administrator.ACHESON\Documents\Microsoft Press\Rs2008sbs\Workspace\Sales\Sales\bin\Debug\AdventureWorksDW2008.rds'

The file exists though. Can anybody advise why the error might be appearing? could it be permissions related?

Cheers

Paul