The Redtron SE should work OK in SONAR. Unzip the Redtron_SE+1.5.zip into your scan path. It will create a Redtron_SE folder containing the Redtron_SE.dll and another Redtron_SE folder. When scanned using default settings the synth shows up in SONAR as
Container (32-bit). This is because the developer did not change the default name provided by Synthedit.
I agree with the recommendations for M-Tron Pro but it is a little pricey. G-Force does have sales but not very often.
Before I picked up M-Tron Pro. I tried a variety of things to get Mellotron sounds. The precursor to Redtron SE was one of them. Redtron use soundfont files based on samples created by Taijiguy found at
http://leisureland.us/audio/MellotronSamples/MellotronSamples.htm around the time the original samples were posted there was a thread in this forum about them
http://forum.cakewalk.com/FindPost/1410821 and BRock created sfz files for the samples. Unfortunately, the links in BRock's post are bad now. It has been some time since I looked at this stuff. I have sfz files for the samples. Some are BRock's original files, some I may have modified and some I am not sure where they came from or if I made them myself. Anyway, I can put them up for download if anyone wants them and/or make them available in the User Contribution thread. FWIW, the Redtron SE modified samples are stored in soundfonts in the Redtron SE folder. They may be played with any soundfont player. I converted the soundfonts to sfz files and added them to Rapture.
I also bought the DSF Mellotron pack for CSC when it was on sale some time ago for $10. The samples are OK. I created my own Rapture programs for them to avoid using the 32bit CSC. I would probably use Rapture Session now. WRT Rapture Session, it might have some tron programs, I do not know because I have Rapture Pro. Rapture Pro definitely includes tron programs. There are some tron-like programs in some of the other bundled synths.
Nori Ubukata's "Art Rock Basics" contains Mellotron samples and presets for Synthmaster. Being sfz based they work in any sfz synth.
Occasionally UVI gives there Mello pack away as a promo item. Not sure I would buy it though.
There is another free pack of samples mention in Deals ATM
http://forum.cakewalk.com/FindPost/3639943 I would grab it and make some sfz files for Rapture but my search for trons ended when I bought M-Tron Pro.