Timeline Maker Pro version 3: 

Version 3, use Timeline Maker's Import Wizard to import files.  You can import Timeline Maker files (*.TLMP, *.TLM) or files from other programs, such as Mirosoft Excel.. These would be saved as a comma  delimited (*.csv) or tab delimited text (txt) files.. 

In addition the Import Wizard allows you to decide where your file's columns import into Timeline Maker's columns, including user defined columns.

Timeline Maker Professional version 2: 

You can import events from a tab delimited text file (*.txt, *.tab, *.asc) or a comma separated value file (*.csv,).

They need to be in a specific format. For more information about importing files, see the help file section titled "Import".