It was described in this topic before.
close this window and open console in Chrome.
Press [ctrl] + [shift] + [i], open “Console” tab, put this and hit [enter]:
if this window will appear again use this on the console:
var input = '<input type="file" name="f_apps_file" class="file_input_hidden" id="f_apps_file" onchange="apps_manually_install();">Install an app manually</div>';
it worked for me many times.