Here is a quick screen capture of where I'm at so far in this remix.

The first stage involved setting up all my samples in time. The vocal was really tricky on this one, and took a lot of playing around with to get it timed properly but perhaps you won't have trouble with it.

Next I was trying to replace the first synth which, as it turns out, is very complex. There are many effects on Tiesto's final version. The files in the remix pack already have been processed with reverb, delays and (too much) side-chain compression by Tiesto and his production team.

Since MIDI was not part of the audio stems I received with this project I had to record it in manually on my MIDI keyboard. Here is a screenshot of what I recorded, click to enlarge:
Piano Roll of MIDI
Next I assigned a synthesizer to this MIDI track in order to turn those notes into sound pressure! A very nice 3 oscilator synth that I go to a lot for this type of modern lead synth sound is Rob Papen's Predator synthesizer (Official Site includes mp3 examples and a demo version). I used a preset as a starting point, first I want to get every sound in place and I'll fine tune all the details as the project progresses. Here is a look at my settings for this part, click to enlarge:
Predator Synthesizer Screenshot

The next thing that I added was a compressor so that I could get the volume of the synth to dip in intensity for a brief moment every time the kick drum hits. This is a technique called sidechain compression, and there are no shortage of tutorials littering the internet teaching how to do this common audio magic in your DAW, in your neighbour's DAW, on Public Transportation, on the way to the Moon and everywhere in between so I won't bore you with the details. Do notice that I have Bus 11 designated for the Kick drum signal in this project. This way, if I wanted to, I could have the kick drum cut out, and keep its pumping effect on the synth. On a dance floor this gives the impression of a beat and causes a lot of anticipation during a breakdown or similar song part, click to enlarge the screenshot:

Logic Compressor Screenshot

The final touch was to alter the EQ. Looking back now it was a questionable decision to mix EQ so early in the song, but either way I ended up strapping on a HPF (high pass filter) at around 18db @ 60hz and a boost of 4db @ 3000 hz, we know for sure that we don't want any noises coming from this synth that are below 100 hz. I'm sure I had a mix reason for the boost at 3 khz but I can't recall right now. If i had a second complimentary instrument that is competing with the synth sound for head room I might apply the boost to this one, and an identical reduction at 3 khz You can check my exact settings in this screenshot:
EQ Plugin Settings

This goes a long way towards explaining what it is that I did, but this video ties up all those words into one simple segment of the remix. Check it out and don't be afraid add your comments.