You can call methods from a dll, if you declare the specified dll. You have to specify the name of the method, name of the dll and list of the parameters. After than you can use the method as a usual visual basic method.
Example:
Declare Function GetNumberOfImagesInTiffFile Lib "BiTiff.dll" ( _
ByVal FileName As String) As Integer
For more information visit to: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconInteropMarshaling.asp