I wanted to add some of my own links to the Main Menu.
After rooting around I figured I had to edit the system_block_mainmenu.html file.
So I edited the file on the server adding the following link just after the main foreach block:
<a class="menuMain" href="<{$xoops_url}>/pcs/Management.php">Management</a>

Nothing happens.

Now, if I then view the template via the system admin/ templates route my new line is not there.
If I open the actual file on the server again my addition is still there...?
Could someone please tell me what am I doing wrong...?

two things you can do:

1. you should ALWAYS modify templates by going to System -> Templates. First thing I do on a new install of XOOPS is clone the default template set and set my clone as the current template set in "System"->"Preferences"->"General Settings".

2. If you do make changes to templates in /modules/<yourmodule>/templates/ you can see them in your site by "updating" your module in "System"->"Modules".

Excellent... I was missing the updating bit.
I also did the cloning thing. Presumably this is done just in case I mess up and need to get back to the default set.

Anyway, thanks for the help... much appreciated.

P.S.
I just looked back at sys admin >> templates and noticed you can actually edit the templates and submit the changes of the clone.
You can only view the defaults.
All is clear.

Also, if you are editing the template a lot and don't want the hassle of having to manually reload and recompile them, check out this thread:

https://xoops.org/modules/newbb/viewtopic.php?topic_id=13890&forum=17