How to install

On Blender, go to File > User Preferences > Add-ons (or Edit > Preferences > Add-ons in 2.80), then press the "Install Add-on from File" button. Select the ZIP file that you downloaded after purchase and install it (choose the appropriate edition for your Blender).
After installed, tick the add-on checkbox to enable it, and save your user preferences so it stays enabled.

How to Use

It's very simple.
You can find it in the Mesh > Vertices menu of Edit mode.
The tool behaves differently depending on what vertices are selected before you start it:

• Select two or more vertices where you want the curve knots to appear and then call the tool — the curve will follow the shortest path between those vertices.
• Select a sequence of connected vertices — the curve will appear along that sequence. Just make sure the selection doesn't loop.
• With nothing selected, call the tool — it goes into the "projection" mode where you can pick points on the mesh surface where the curve knots should appear, similar to how the mesh Knife tool works.
  
  

In any of these cases a curve will appear. You can grab / rotate / scale its control points as much as you want, then use Enter to confirm or Esc to cancel.

There are some actions that you can use while the tool is on:

• Curve Grab: this is a special way of adjusting the curve without having to move its handles: you hold Ctrl and click to "grab" the curve at that location, then move the mouse the change it.
• Use Extremes: toggles the deformations caused by the start and end control points of the curve. This is best used when either of the ends of the curve starts from a sharp turn of your mesh. .
• Use Direction: toggles the use of the curve orientation (tangent) to deform the mesh, in addition to using its position. It makes the geometry "rotate" with the curve.
• Settings Menu: brings a pop-up menu where you can change some tool options and call some other actions (the tooltips are helpful in here).
• Increase / Decrease Falloff: just like the Proportional Editing feature of Blender, this lets you control how the curve affects nearby vertices. In the Settings Menu you can choose the falloff mode (Off / Distance / Connected) and the falloff type (Smooth / Linear / Constant etc.), all these settings control how the curve affects the geometry.
• Unbind Curve: enters a mode where you can adjust the curve without it affecting the mesh. This way you can subdivide the curve, change the knot handle types (default key V) etc. This all happens in curve Edit mode.
  Then you can use this same action to toggle back to deforming the mesh with the modified curve.
• Twist: this is a Blender action (default keymap Ctrl + T), it lets you twist the knots of the curve. If you turn on "Use Rotation" you can use this to twist the mesh. Use Alt + T to clear the tilt.
• Shrink/Fatten: this is also a Blender action, it lets you increase the radius of the knots of the curve. This lets you inflate the mesh.

All of those actions (except the last two, which come from Blender) have customizable keymaps that you can edit in the...

Add-on Preferences

In here you can change some global tool options as well as the keymaps for all those actions.

Preferences for v0.8.8
 

• Curve Distance: lets you set a distance to offset the curve away from the mesh, as if hovering above it.
• Force Primary Vertices: the primary vertices are any vertices that you selected before calling the tool, as well as the vertices in the shortest path between them. When this option is on, these vertices always get 100% of influence from the curve. Sometimes this doesn't look right or you want a softer effect, so you can turn this off.
• Curve Handle Visibility: controls what should happen with the curve handles when you start the Ctrl + Click grab mode.
• Menu Type: mostly cosmetic, lets you change the type of pop-up menu from a dialog menu to a pie-style menu.
• Set Curve Color: this option temporarily changes the "Wire Edit" theme color to a color of your choosing. Use this when the curve is somehow difficult to see.