1 . Which version of Unity do you have ?
If you have an "old" version of Unity, you may need to change the compatibility mode of the engine in Designer.
To do this, in the top menu of Designer 6, go to Edit->Preferences->Projects. Here you will find the option 'compatibility mode'. Change it to 'Substance engine V5'. Hit 'ok' and now export your sbsar.
But be careful, if you see yellow highlighted nodes in your graph, it means that those nodes are not compatible with V5 engine. It often happens with the normal map node.
If this node is highlighted, change the 'Alpha Channel Content' option for 'Fill Alpha With Input'
2. to open a sbs made in Designer 5 in 6, just drag and drop it in Designer 6. Your graph will be updated. But you won't be able to open it in Designer 5 after this.