How to make file manager load in N1ED
N1ED is an editor based on TinyMCE or CKEditor 4 (you can choose) but has all the required plugins installed to start working immediately. Flmngr is one of base parts of N1ED and if you have enabled N1ED you should see Flmngr there.
In case you are using N1ED as a plugin for an existing TinyMCE/CKEditor installation, please ensure that you have enabled it (in CKEditor: add extraPlugins: "N1ED-editor", in TinyMCE: add plugins: "n1ed").
If you are certain that N1ED is loaded, but you still do not see the file manager buttons, this can only mean that you have switched to the Manual toolbar, but did not add any buttons to your toolbar config. Please add the required buttons or switch back to the Modern or Classic toolbar instead of using the Manual one. After you have set this option, ensure that you have not hidden these buttons from displaying (they should not be grey in the Dashboard toolbar configurator).