Use Cuttermaran to trim your M2V and Audio file before authoring in DVD Lab. In Cuttermaran, set your in and out, and range to cut list, then cut video/audio. Now use the trimmed A/V in DVD Lab.
Before you author your DVD, you need to have all of your assets ready for authoring, as DVD Lab is just for authoring. It has some minor editing functions, but life is easier if everything is ready to author from the start of the project.