Does the filename change each time? If so, it's going to be difficult to link to it dynamically.
On the other hand, if it doesn't change each time then the users could just have shortcuts to the Excel file on their desktops.
Regardless, if you create a blank report, it doesn't even need a datasource, you can create a text object in the report header - "Click here for latest Excel file".
Then right click on the text object and select Format Field. The last tab is titled Hyperlink - on that tab you can select File as the hyperlink type, and browse to the file location. Now the text object will open the file when clicked, you may want to format the text blue to make it obvious that it's a link.