>Everything seems to be properly formed - can anyone cast some light on what I am missing here?

Sure, the problem is not in create table, but in the trigger - you need SET TERM since you cannot use ; inside the trigger if it is also the terminator:

CREATE TABLE "Administrators" (
"_Id" INTEGER NOT NULL,
"_MyDetailId" INTEGER DEFAULT 0,
"_Name" VARCHAR(50),
"_Designation" VARCHAR(50),
"_Address" BLOB SUB_TYPE TEXT SEGMENT SIZE 255 CHARACTER SET ASCII,
"_Emails" BLOB SUB_TYPE TEXT SEGMENT SIZE 255 CHARACTER SET ASCII,
"_TelephoneNumbers" BLOB SUB_TYPE TEXT SEGMENT SIZE 255 CHARACTER SET ASCII,
"_Reference" VARCHAR(50)
);


ALTER TABLE "Administrators" ADD CONSTRAINT "PrimaryKey" PRIMARY KEY ("_Id");
CREATE INDEX "_Id" ON "Administrators"("_Id");
CREATE INDEX "_MyDetailId" ON "Administrators"("_MyDetailId");


CREATE GENERATOR "GEN_Administrators__Id";

SET TERM ^^ ;

CREATE TRIGGER "TR_AI_Administrators__Id" FOR "Administrators"
ACTIVE BEFORE INSERT
POSITION 0
AS
BEGIN
IF (NEW."_Id" IS NULL) THEN
NEW."_Id" = GEN_ID("GEN_Administrators__Id", 1);
END ^^

SET TERM ; ^^

HTH,
Set