Does it matter if I created a BO project in the solution because on the second try I just added a BO and not a BO project and then a BO to that project?
No. This will not make a difference. If I had to guess, I would assume that the first time you did not create an SF application but just a standard .NET application which does not have the AppMain, references, etc. that the SF app expects. In any case, when designing an application, it is always best to store BOs in a class library anyway instead of the entry point executable. This will give you more flexibility when creating other assemblies or scaling your application.