Hector-- I assume you are doing a partial boil. If so, I believe you should try to match the boiling wort gravity with the OG of the topped off wort, so hops utilization is consistent with the recipe. Thus if your partial boil volume is, say, half of the final volume, add about half of the DME for the 60 minute boil and half at or near flame out. If you are boiling smaller volumes, add less DME up front and more toward the end of the boil.
I do many partial boils, and the late addition is always difficult to dissolve, as Yooper said. There is also a slight break that occurs about 5 minutes after the addition, so I try to time this addition to allow this break to occur.
If you have the ability to do a full boil, I don't think there is really any need to do late addition for DME as Rev2010 points out.