By default, the Metadata Editor will always appear at export time. In Audacity 1.3.7, the editor appears before the Export window. Enter any metadata you require in the editor, then click OK (not "Save") to proceed to the Export window. The Export window lets you choose the file name, path and any custom encoding settings (by clicking the Options button).
To prevent Metadata Editor appearing at export time, click Edit > Preferences, then the Import / Export tab, and in the "When exporting tracks..." section, uncheck "Show Metadata Editor prior to export step". The tags can still be viewed or edited at any time prior to export at File > Open Metadata Editor, and the tags in the editor at export time will still be exported