Matjaž:
Each version of ArchiCAD only saves back one version, so to go from ArchiCAD 13 to 11, you need to Save As... in 13 to 12, then open that file in ArchiCAD 12 and Save As... to 11. If you don't have 12, then talk to your local reseller (the dongles work with previous versions). Erika's advice on library parts stands: ArchiCAD 11 will not be able to read 13 library parts, and there is no easy way to change a library part to an earlier version.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David