I'm using Qt5.3+Qt creator3.3.0 on max os x 10.10.1

While debugging in creator,I can see "Locals and Expressions" in debug view,but failed to modify a local variable's value(a simple type:int) and another variables's value.
As a IDE,qt creator must have provided this feature, so there may be something wrong with my creator or there are some configure to be done before using this feature.
Can anyone help?
Thanks in advance.