I'm afraid the lens flares could have been a big plus in Max but are in fact quite the opposite. You can't blur them. There are two ways of doing lens flares in Max:
1: VideoPost - here you have a lot of neat features but then again somewhat unflexible.
2: Lens Effects Panel - here you have fewer options yet a few others that VideoPost doesn't have.
3: If you only need round flares you can do them with volumelights (omnis withou shadowcasting, with far attenuation on and inverse decay on - the latter is very pivotal so note that one - and have two of them on top of each other, one of them smaller and maybe more white? 'pending on which colours you want)
There is another solution which I like but costs money
And that is to get a hold of Cebas' Pro Optic Suite - you won't regret it and you will be in lens flare heaven. I MEAN IT!