I don't think it is possible to use ACS with Lotus notes for user authentication. These are the external databases supported with ACS.

a) Windows User Database

b) Generic Lightweight Directory Access Protocol (LDAP)

c) Novell NetWare Directory Services (NDS) when used with Generic LDAP

d) LEAP Proxy Remote Authentication Dial-In User Service (RADIUS) servers

e) Token servers

f) Open Database Connectivity (ODBC)-compliant relational databases (ACS for Windows)