Try setting the server to "localhost" (lose the ":5432" port bit).

I'm pretty sure 5432 is the default port, anyway.

Sean

When I clicked the install button at the end I got the following error... :S

Setting up 'mysite/_config.php'...
Setting up /Applications/MAMP/htdocs/SilverStripe/mysite/_config.php
Setting up '.htaccess' file...
Setting up /Applications/MAMP/htdocs/SilverStripe/.htaccess
Building database schema...
[Warning] pg_query() [function.pg-query]: Query failed: ERROR: relation "SiteTree_versions" does not exist LINE 2: FROM "SiteTree_versions" GROUP BY "RecordID", "Version... ^
POST /SilverStripe/install.php

Line 174 in /Applications/MAMP/htdocs/SilverStripe/postgresql/code/PostgreSQLDatabase.php

Source

165       if(isset($_REQUEST['previewwrite']) && in_array(strtolower(substr($sql,0,strpos($sql,' '))), array('insert','update','delete','replace'))) {
166          Debug::message("Will execute: $sql");
167          return;
168       }
169
170       if(isset($_REQUEST['showqueries'])) {
171          $starttime = microtime(true);
172       }
173
174       $handle = pg_query($this->dbConn, $sql);
175       
176       if(isset($_REQUEST['showqueries'])) {
177          $endtime = round(microtime(true) - $starttime,4);
178          Debug::message("\n$sql\n{$endtime}ms\n", false);
179       }
180       

Trace

pg_query(Resource id #753,SELECT MIN("ID") AS "ID", "RecordID", "Version" FROM "SiteTree_versions" GROUP BY "RecordID", "Version" HAVING COUNT(*) > 1)
Line 174 of PostgreSQLDatabase.php
PostgreSQLDatabase->query(SELECT MIN("ID") AS "ID", "RecordID", "Version" FROM "SiteTree_versions" GROUP BY "RecordID", "Version" HAVING COUNT(*) > 1,256)
Line 127 of DB.php
DB::query(SELECT MIN("ID") AS "ID", "RecordID", "Version" FROM "SiteTree_versions" GROUP BY "RecordID", "Version" HAVING COUNT(*) > 1)
Line 303 of Versioned.php
Versioned->augmentDatabase(,,,,,,)
Line 963 of Object.php
Object->extend(augmentDatabase,)
Line 3012 of DataObject.php
DataObject->requireTable()
Line 209 of DatabaseAdmin.php
DatabaseAdmin->doBuild(1)
Line 1026 of install.php
Installer->install(Array)
Line 190 of install.php

Thank you again Sean! That worked!