The block of code inside the IF & ELSE statement should be within BEGIN and END block. Like this:
IF @key IS null
--address book doesnt exist just so create default
BEGIN
INSERT INTO tbl_AddressBookMaster ([addMaster_UserId], [addMaster_Title], [addMaster_Description], [addMAster_NoRecipients])
VALUES (@tmp_custID, 'Default', @tmp_Description, 0)
SELECT @newSignUpID = SCOPE_IDENTITY()
--add addresses to second table
INSERT INTO tbl\_AddressBookAddresses (\[adds\_ABMId\], \[adds\_Email\], \[adds\_RecipientForename\], \[adds\_RecipientSurname\])
VALUES (@newSignUpID, @tmp\_Email, @tmp\_Forename, @tmp\_Surname)
** END**
ELSE
--default already exists
**BEGIN**
--add addresses to second table
INSERT INTO tbl\_AddressBookAddresses (\[adds\_ABMId\], \[adds\_Email\], \[adds\_RecipientForename\], \[adds\_RecipientSurname\])
VALUES (@key, @tmp\_Email, @tmp\_Forename, @tmp\_Surname)
**END**
Niladri Biswas