This variable amongst others is in language pack... /lang/eng.php or whatever; button itself is in templates/user_login_only_form.html.
We are a bit tired of the questions like this, so here is the short basic solution how to know where to edit this and that, if you want to edit it.
Everything what you need is to search within extracted miniBB files for content
. You must know only what part of content is to search about. In your case, part of content will be "$l_enterforums", or at least, "l_enterforums".
For Windows, install Total Commander
, browse into directory where miniBB files are exctracted, run Alt + F7 and find all files by "file text". For other OS you could find the similar file manager with ability to search by keyword within files.
Click on the thumbnail below to enlarge and view full screenshot of this panel.
On results page, click a file name and open it via pressing F3. Press F3 even further unless you reach the code which contains the searched fragment.
Default windows "Search" (Start -> Search -> For Files or Folders -> All Files) gives the same results, but only not allowing to search within specified directory (or at least we don't know how). TC is a great tool not only for this, but also for many other file management and FTP operations.
P.S. This is not TC advertisemet, we are not affiliated with them. It's just a great software and free to download.
Thank you for giving us the time for more dedicated work than answering repeated basic questions :)