The App by default protects you against editing any of the built-in patterns or any rhythm that was downloaded from the online database. Instead of editing it directly, tap the + button to add a new pattern, you can than make the new pattern a copy of an existing one (or blank) and edit that.
This was a problem with Beat Vibe in versions prior to 2.0. The built-in slider of the iPhone OS is has a very small sensitive area and the touch screen is sometimes not accurate enough. The result was that catching the knob of the slider was very difficult, this happens with other application that use the slider control, even Apple's own.
As of version 2.0 the slider code was re-written to override the default behavior and is now easy to select. if You are using a prior version, please update to the latest version.