1-Right click on Advance stell icon & choose "Properties".
2-Click on "Shortct".
3-In "Target" box after .dwt" leave two spaces & type /AcadVer R20.0\ACAD-E001:49 & click on Apply.
This will force Advance Steel to be open only with Acad 2015.
Hope this will help. If you need more help drope me an email.