You can list available ODBC datasources from within PowerBuilder. You need to declare the following external functions :

FUNCTION integer SQLAllocEnv(ref long henv) LIBRARY "odbc32.dll"
FUNCTION integer SQLFreeEnv(long henv) LIBRARY "odbc32.dll"
FUNCTION integer SQLDataSources &
(long henv, integer idirection, ref string szdsn, int idsnmax, &
ref integer idsn, ref string szdesc, integer idescmax, ref integer idesc) &
library "odbc32.dll"

The following snippet will initialize a DropDownListbox with DataSources defined on the current workstation.
long ll_henv
string ls_dsn, ls_desc
integer li_direction, li_dsnmax, li_dsnlen, li_descmax, li_desclen, li_rc
integer li_length = 255

ls_dsn = Space(li_length)
li_dsnmax = li_length
ls_desc = Space(li_length)
li_descmax = li_length

IF SQLAllocEnv(ll_henv) = -1 THEN
    MessageBox("SQLAllocEnv", "FAILURE")
ELSE
    li_direction = 1
    DO WHILE SQLDataSources &
        (ll_henv, li_direction, ls_dsn, li_dsnmax, li_dsnlen, &
         ls_desc, li_descmax, li_desclen) = 0
         ddlb_1.AddItem(ls_dsn + "  [" + ls_desc + "]")
    LOOP
    SQLFreeEnv(ll_henv)
END IF