In addition to their mount capabilities, all engines are surface attachable to facilitate cluster construction. There are four available adapters, all with built-in interstage fairings: 1.25m Ultra-Light Interstage Cluster Adapter (for landers), 2.5m Ultra-Light Interstage Cluster Adapter (for landers), 2.5m Cluster Plate, and 3.75m Cluster Plate. Along with these, there is also a large 3.75m mounted Expanded Cluster Tank with a large base for attaching clusters and carries fuel, but cannot be stacked.
Included in the download are optional high, medium, and low resolution textures. Medium resolution textures are installed by default.
Q: What do I do with this pack?
A: As you might guess, make engine clusters. Attach a cluster plate to the bottom of your fuel tank (or don't, if you like weird looking floating engines), then attach engines. The standard and lightweight adapters can all be included in stacks.
Q: Do these replace stock engines?
A: No. There will situations where a single stock engine suits your rocket better. These are only intended to help remove some of the limitations of stock propulsion. They are also not intended to be "stockalike", though they have fairly similar statistics to their stock counterparts.
Q: This is too easy!
A: Don't use it! 
Known Issues:
The tank butt adapters may (very probably) vanish if you change scenes and then return to your craft. This is a stock bug with the jettison module used to create the hidden adapters.