Go to the properties of the track.
The 2nd tab is called something like "Indexes, limits, splits".
Nero is quite slow if you "zoom" a lot to have good precision. Feurio is more comfortable for that job.
Latest versions of Nero 5.5 are also buggy if you save your compilation with splitted tracks. The compilation is corrupted when you open it again later... so, do the job, burn it before closing Nero, and forget about using it again another day if you have one of those buggy versions.