Jalatext load only a limit account of lines from the text file for not consuming all the memory. So, for navigating within the file, you have to load sections of the file. To do so, you can use the operations from menu:
In the status bar you can view the current position in the file, and the amount of lines loaded. You can change the number of lines loaded in memory in the Preferences window.
You can go directly to the begin or to the end of the file, selecting Navigate->Go to Begin of File and Navigate->Go to End of File respectively.