What you need to compute travel times are seismic models. TTBOX stores models in ASCII files in formats named .clr (Continuous LayeR) or .nd (Named Discontinuity) which are described in the section on File Formats.
The .nd format stores models in the way that is used by the computation routines: It is a tabular format which defines velocities, densities and Q factors at discrete depths.
The .clr (pronounced "clear") format is designed to store models which are defined piecewise continuously by Layer Polynomials, like IASP91 and PREM. It is a pure storage format and output of MKREADCLR needs to be transformed in order to be usable for computation (see section on conversion between CLR and ND).
To get you started, the subdirectory "data" of the TTBOX distribution contains several .clr and .nd files. They may also serve as examples for the file format. These files are:
These files are readable with MKREADCLR and MKREADND, respectively. See sections "Read Model from .clr File" and "Read Model from .nd File" to learn how.
The data directory contains some additional files with lists of published travel times for IASP91 and JB models. These files were used as reference data during software validation.