When you do not explicitly specify that TRANSACTIONAL=0 in your CREATE query, everything works fine:

CREATE TABLE table_a (col_1 INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, PRIMARY KEY(col_1)) ENGINE=MEMORY;
Query OK, 0 rows affected (0.018 sec)

ALTER TABLE table_a ADD COLUMN col_2 VARCHAR(255) AFTER col_1;
Query OK, 0 rows affected (0.022 sec)

If you do specify TRANSACTIONAL=0, CREATE TABLE throws a warning, and ALTER TABLE fails:

CREATE TABLE table_b (col_1 INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, PRIMARY KEY(col_1)) ENGINE=MEMORY TRANSACTIONAL=0;
Query OK, 0 rows affected, 1 warning (0.020 sec)

SHOW WARNINGS;

Warning

1478

Table storage engine 'MEMORY' does not support the create option 'TRANSACTIONAL=1'

ALTER TABLE table_b ADD COLUMN col_2 VARCHAR(255) AFTER col_1;
ERROR 1478 (HY000): Table storage engine 'MEMORY' does not support the create option 'TRANSACTIONAL=1'