The envelope section of a synthesizer is a little bit complex, and it takes experimenting to grasp it.
First of all, attack, decay, and release refer to time, and sustain refers to amount. Attack concerns the initial strike of the note and how long it takes to rise to its peak. Release is just the opposite - how long it takes for the note to entirely fade away after you've released a key. Sustain refers to the level at which a tone will remain when you hold a note, and decay refers to the length of time it will take to fade to that level.
Envelopes are used primarily to control the VCA (amplitude/volume) and the VCF (filter/timbre). On the Rev2 these two sections are called "Low Pass Filter" and "Amplifier," respectively. There are other uses for envelopes, but amplitude and filter are the main two.
Based on the setting of the VCA envelope, the volume of a sound will rise (attack), fall when the note is held (decay and sustain), and then completely fade away when the key is let go (release). For this the "Cutoff" parameter has to be at least partly turned up.
Based on the setting of the VCF envelope, the filter will open (attack), gradually and partly close when a note is help (decay and sustain), and entirely close when the key is let go (release). For this, the "Env Amount" parameter has to be at least partly turned up.
The VCA and VCF envelopes are used together to create all sorts of complex note behaviors. The best way to keep things simple for now is to set one identically to the other.
That's a quick introduction to envelopes. Regarding your actual question, I can only guess that, if changing the attack time has no effect, then it's because the release time is too long and the note is still sounding. Try turning the VCA and VCF release times to "0," and then adjust the release. Then you should hear the difference. Also, set the "Delay" parameters to "0".