There is a bug in the Hidden Services Manager which prevents part of the UI from being shown.
There are multiple workarounds available.
Any one of these three workarounds will resolve the issue for now.
1: Use the Hidden Services Manager UI from /i2ptunnel/ instead of /i2ptunnelmgr
2: Use the "Open in New Tab" link(A square with an arrow) at the top of /i2ptunnelmgr
3: Disable JavaScript
Sorry for the bug, it will be fixed soon.