may i know the extension name ? and your qliksense desktop version? (note: if u r using older version of sense desktop then some the latest extension won't support - double check that installation steps in the extension ) the extension that u placed is zip file? if so then unzip/extract & place it respective folder (refer below screen shot)
1) downloaded the extension ( make sure that u r qliksense desktop supported or not. always downloaded latest version) for example u can download this
note: while u can download any extension from Qlik branch/GitHub site, there will be installation & configuration steps just follow that
2) unzip and place under below location
3) try to close and re-open your qliksense desktop and verify the extension that we added above.