If it is just another project to which you want to add reference, try adding the project to the solution and then add reference to that project in the solution.
If the above is not the case and if it is native dll then you can use
DllImport Attribute.
Else, follow these links which may be useful
using regsvr32[
^]
using TlbImp.exe[
^]
Using DllImport[
^]
Hope, it helps :)