DLL not found error

Okay, I tried to scrap the furom for an answer but none specific for my issue...

 Basically, I using VS 2005 and have e a thrid part DLL in my Bin folder... I have another class, say fooClass ,which is located in app_code folder. I make a call like this

 *****foolClass.vb

Public class fooClass

public declare function open lib "dllFIle.dll" (byval StrFileName as string) as string

end class

******Default.aspx

foo.open(filename)

*********
I get and error  "unable to load dll module can not be found." I have set the reference to the dll. Any answers ??  ...