Using Excel Add-In for SAP BO generates Error Runtime 1004

Even though you had registered all the .xll files and the relevant .dll files in would suggest you to once register the XLL (specifically) in VBA, through Files -> Options -> AddIns.

Once this is done, check the dependency of the relevant XLL using the Dependency Walker. Only in case you find all the relevant DLL are in place your VBA will be import to import the relevant library. In adverse case, check the path of the DLL from the dependency walker and if found incorrect, place the DLL at the correct path to overcome the issue in concern.