A ToolBook project file is saved in a file with a .TBK extension. In order to open a .TBK file in order to edit it, you will need to own/purchase the full Authoring Version of ToolBook.
However if you don't own the Authoring version of ToolBook but still need to open a .TBK file simply to use/view it, the Runtime version of ToolBook may be able to be used.
For each version of ToolBook, there is a specific Runtime engine, which knows how to use/view files from that version of ToolBook. If, for example, you don't own the Authoring version of ToolBook 11 but have a .TBK file from ToolBook 11, you'll need to download and install the ToolBook 11 Runtime in order to use/view it.
The tricky part is knowing which version you need. There is no way to know, by looking at a .TBK file, which version it came from, since they all have the extension of .TBK. You'll need to ask the creator of the file, to find out which version of ToolBook it was created with.
Even if you install the correct Runtime version of ToolBook, you may encounter problems with missing audio/video. A .TBK file contains just the textual and graphical parts of the project. If the project contained any audio or video, those would be stored external to the .TBK file. If you only have the .TBK in your possession and are trying to open it, ToolBook may very well complain at some point that some externally located audio/video file is missing.
For this reason it is not really recommended that you take an arbitrary .TBK file you find and simply try to open it. It is always a better idea to find the creator of the file and find out what is needed in order to view the file properly (which version of ToolBook, and where to locate the needed external audio/video files).
For these versions of ToolBook, the Runtime installer was combined with the Neuron installer. As such there is no longer a separate installer just for the Runtime Engine. If you would like to install the Runtime files on a computer, use the Neuron Installer. The Neuron Installer actually is identical to the older runtime installer but with a minor additions in that it installs the full runtime engine but also installs a few components (plugins) which know how to communicate with the browsers. To locate the correct Neuron Installer for your needs, please click here.