That's because you're probably using an older version of Zbrush that doesn't have dynamesh (or the slice curve tool). Z4R2 is the latest version (with Dynamesh), though an even newer version will be out in less than a week.
Slice Curve works great if you're using Dynamesh+Groups. Otherwise there is a wide range of selection tools that you can combine with Tools: Geometry: Del Hidden. Works only on the lowest subdivision.
Dynamesh is in ZB4 r2. You probably need to upgrade. Its a free upgrade btw. We still need pictures though. We need to know what the mesh is and what you want to achieve.
You seem to ask alot of questions about quite simple tools in ZBrush. I know ZB is quite hard to learn but theres tons of tutorials out there. Watch all the vids at ZBclassroom. Visit ZBC tutorial section. I strongly recommend Eat3D's hard surface 2. To answer your question, it all depend what you want to do. Do you simply…