The container is AT the posted coordinates.
In order to claim a find, in addition to signing the log, you must have found at one (1) other geocache.
The rules are simple and are as follows:
1. All cache types count as do finds from all years.
2. You must find the physical container and sign the log to claim a find.
3. You can use the Project-GC Challenge checker to confirm you qualify or check your progress.
4. Found it logs by users who do not qualify may be deleted without warning. However I do not anticipate this happening often, if ever.
If in doubt (unlikely, but still) you can use this challenge checker to check your qualifications.

The easiest challenge I ever encountered was this one, but zero finds is apparently too easy to publish under the current guidelines.
This is a challenge cache. A challenge cache requires seekers to find an associated physical cache, and to find an additional set of geocaches as defined by the challenge owner. Challenge caches encourage geocachers to set and achieve fun goals. Examples of challenge caches: find a cache every day of the calendar year, or find one for every Difficulty/Terrain combination.
You may sign the logbook at any time, but may only post a "Found It" log after you have:
- Logged qualifying geocache finds; and
- Signed the logbook
You may post a note at any time and may wish to do so if you sign the logbook before finding enough qualifying caches or if you have the qualifying finds but have not yet signed the logbook.