Congratulations, your first plug-in 
Generate plug-ins will return audio to the selected track if there is a track selected, but will create a new track (so that there is somewhere for the generated audio to go) if no track is selected. When you use it on your project, ensure that at least one track is selected. You don’t actually want a selected region, you just need to click in the track at the point where you want the audio (silence) inserting.
An easy way to insert the same amount of silence at the very beginning of all tracks is to press Ctrl+A (select All), then click the Home key (cursor to the beginning), then run the plug-in.
You can toggle whether a track is selected or not by using the up/down cursor keys to move to the appropriate track, then press the Return key to toggle the selection.
We generally add a line indicating the software license terms of plug-ins. For example, if you want the plug-in to be open source, just add something like
Released under GPL v2
into the ;info line, and add a comment below the header to say:
;; Released under terms of the GNU General Public License version 2
There’s some information about “conventions” that we’d like to encourage plug-in authors to adopt here: Conventions for Nyquist Plug-ins Of course there is no compulsion to follow these suggestions, but consistency between plug-ins will be helpful to both users of the plug-ins and to anyone that is looking to learn from the code. You are also welcome to add your own comments to that topic if you wish.
I notice that you’ve named the plug-in “Generate Latency Offset”. In Audacity 1.3.x there is already an automatic way to do this. See here: http://manual.audacityteam.org/man/Latency_Test