If you are having serious issues with the solder sticking, try this.

Clean the area in question on the motor can with emery cloth and rinse with alcohol. Use a very hot iron and rosin core solder. Melt a small dab of solder to the tip of the iron. Press the iron to the can (the small drop of solder helps increase the heat transfer.) Apply the solder to the can (not the iron) about 1/10 of an inch away. As soon as the can is hot enough the solder will flow. This should stick. Once you have primed the area with solder, it will be easy to stick the capacitor leads to that area later.

Besides cleaning the area well, the best tip I can give is use a very hot iron. Crank it up as high as it will go. The can acts as a big heat sink. If the iron is too cool or can't deliver heat quickly it takes too long to heat up a spot to the point where the solder will flow. While you are waiting the heat will travel about the can making it difficult to handle and possibly melting the plastic parts. The result is a cold solder joint. The solder flows around the iron but never really attaches to the can.