How to solve PostgreSQL pgAdmin error "Server instrumentation not installed" for adminpack?

Question 1

For current versions of PostgreSQL and pgAdmin, the "Guru" dialog warning has a "Fix it!" button or command. Use it.

If there's no "Fix it!" then we can use the Unix command line as follows.

This is for PostgreSQL 9.1. Older versions do it differently.

PostgresSQL docs are here:

Install adminpack like this:

$ sudo apt-get install postgresql-contrib

To verify we got the files, list them:

$ dpkg -L postgresql-contrib-9.1 | grep adminpack

Result:

/usr/share/postgresql/9.1/extension/adminpack.control
/usr/share/postgresql/9.1/extension/adminpack--1.0.sql
/usr/lib/postgresql/9.1/lib/adminpack.so

Alternate way to find the adminpack files:

$ sudo updatedb
$ locate adminpack

Use psql to create the extension:

$ sudo -u postgres -i
$ psql [dbname]
# CREATE EXTENSION adminpack;

(If you don't have super-user or if you need to create a per-db extension, see the comments below by @w00t to use \c dbname to connect to the database)

To verify:

# select * from pg_extension;

Result:

extname  | extowner | extnamespace | extrelocatable | extversion | extconfig | extcondition 
-----------+----------+--------------+----------------+------------+-----------+--------------
plpgsql   |       10 |           11 | f              | 1.0        |           | 
adminpack |       10 |           11 | f              | 1.0        |           |

To load the extension into pgAdmin, see the database server icon:

Right-click the icon then choose "Disconnent"
Right-click the icon then choose "Connent"

To verify adminpack is working:

Click a database icon
On the top-right pane, click the "Statistics" tab.
Scroll to the bottom of the Statistics.
You now see a "Size" entry that shows the database size on disk.

Question 2

The "Fix It!" button would appear in the "Guru Hint" dialog next to OK and Cancel. If you are not offered the button, enter the following in a console:

sudo apt-get install postgresql-contrib

then click the guru button (in my version, a face to the left of the ? button) and the "Fix It!" button should appear. Click it.

See the answer from joelparkerhenderson if the Fix It! button doesn't appear.