i have the same issue - mysql5 is installed (64bit afaik) and it cannto find any DB of any kind:
Checking available databases
Check which of the supported extensions are installed. At least one of them is required.
MySQL: NOT AVAILABLE(Not installed)
MySQLi: NOT AVAILABLE(Not installed)
PostgreSQL: NOT AVAILABLE(Not installed)
SQLite (v2): NOT AVAILABLE(Not installed)
we are using mysql for other things on the same localhost and i have also installed hmail/roundcube combo on other servers and havent come across this issue.
sys info:
server 2003 R2 std 64bit
mysql5.1.31-community