Possibly save 1 hour of your time: This shows how you can add an expression to use as parameter for the Flat File Connection Manager or any other ones.

Right click on your connection manager and choose Properties. The properties window will show up. Find the Expressions and then click on the ellipsis on the right.

In the Property Expressions Editor that opens, you can now choose different Property such as ConnectionString. In my case, I set it to a user variable that tells me where the error log file is.