Suresh
It depends on what/how you have written the addin?
To add the file in, goto
File, options, Addins
Manage Addin's, Go
Use the Browse button and find your addin
Apply
If you wrote a Function you should now be able to use it in any file by simply =myFunction(x, y, z) or however you coded it
If it has Subroutines, they can be called from another module using:
Call mySubroutine(x,y,z)