The type or namespace name 'business_logic' does not exist in the namespace

I am trying to run a web site project, where its used 3-tire architecture. The there are 3 other class libraries used to create reference dll's which are used in the project.
Those 3 dlls built successfully but when I add reference of those by
1.right click web site path(in soln explorer)-Add reference-browse (class librarie's 'bin' folder...'<--.dll>')-click ok____for all 3 dlls.
here I got the error message...

Error    3    The type or namespace name 'business_logic' does not exist in the namespace '<--->' (are you missing an assembly reference?)

even I tried ->
2.Right click web site path(in soln explorer)-Add reference---Project---path of the .dlls

this gives an error message....

There is no file <.dll name> at

Please tell me what to do....Thanks in advance.