When there is application trial feature enabled and there is serials text filed provided and built into executable calling mdm.Application.Trial.activate(...) method with correct serial key should remove trial protection and change results that are returned for example with mdm.Application.Trial.usesLeft property.
The expected change is that value returned by "usesLeft" after activate is either non number value (NaN) or value of -1.
In current release activate seems to not have any affect either on what is returned by calling mdm.Application.Trial.usesLeft on application startup or after activate was called.

To repeat please download and compile provided sample project.
It's code is based on below script:

//
//
//
import mdm.*;
//
const SERIAL_CODE_IN_SERIAL_TEXT_FILE:String = "abcd";
//
function initApp():void
{
// #1 check results on application startup
var numOfUsesLeft:Number = mdm.Application.Trial.usesLeft;
// #2 NaN (or at least -1) should mean "trial protection was removed"
debug("num of uses left: "+numOfUsesLeft+" at startup");
// #3 evaluate/validate serial key (provided with setup in ZINC Builder)
var isValidKey:Boolean = mdm.Application.Trial.evaluate(SERIAL_CODE_IN_SERIAL_TEXT_FILE);
debug("key: "+SERIAL_CODE_IN_SERIAL_TEXT_FILE+" is valid: "+isValidKey);
//
if(isValidKey)
{
// #4 remove trial protection feature after checking serial code validity
var trialRemoved:Boolean = mdm.Application.Trial.activate(SERIAL_CODE_IN_SERIAL_TEXT_FILE);
debug("trial removed: "+trialRemoved);
}else
{
// ops, in test that should be always correct
debug("incorrect test setup");
return;
};
// #5 display what is currently returned by mdm.Application.Trial api
numOfUsesLeft = mdm.Application.Trial.usesLeft;
debug("num of uses left: "+numOfUsesLeft+" at end");
};
//
var tf:TextField = null;
function debug(msg:*):void
{
if(!tf)
{
tf = new TextField();
tf.width = stage.stageWidth;
tf.height = stage.stageHeight;
addChild(tf);
};
tf.appendText(msg+"\n");
tf.scrollV = tf.maxScrollV;
}
//
mdm.Application.init(this, initApp);


and one should see results similar to:

num of uses left: 998 at startup
key: abcd is valid: true
trial removed: true
num of uses left: NaN at end

(or)
num of uses left: -1 at end

The actual results are:

num of uses left: 998 at startup
key: abcd is valid: true
trial removed: true
num of uses left: 998 at end