I ran into similar issues using QGIS. I am definitely a 'newbie', but this worked for me using QGIS 2.10.1-Pisa, maybe it will help others who are new to QGIS:
- Open the "Advanced Interface" from the lower right hand corner
- Open the Advanced Digitizing toolbar: View -> Toolbars -> Advanced Digitizing
Select the Edit button from the normal Digitizing toolbar (the pencil)
You should see a red X at each vertex in your feature.
Select the Split Features tool from the Advanced Digitizing toolbar.
Now you're ready to do the split:
- Left-click once on the first side of the feature you want to split
- Left-click on the other side of the feature.
Right-click to confirm the edit.
Once or twice I was not careful about the layer I was looking at vice the layer I had selected for edit. It's actually easy to have multiple layers in your layer list, have 1 selected for display and a different layer selected for edit. In that case, no matter how hard you try, your edit's are unlikely to work. Although it looks to you as if you are editing the layer you are looking at, you are really editing the layer selected for edit in QGIS, which do not have to be the same thing.
At first I spent a lot of time trying to ensure I was clicking right on the line when I wanted the split to occur. You don't have to. The split line has to at least include the line you want to split, but it can extend beyond it - and this is actually much easier most of the time.