This is a guide I posted in another thread to help a member with his design, I thought it would be wise to post it here also.

Here's what I put together to join a curved edge seamlessly to a flat surface.

First use the Line tool make the the lines on the cylinder for the 2 straight edges for the curve.

Then you use the Curve tool and contect the 2 lines and this should create a new curved surface.

Next select the outline of the top circle on the cylinder then click on Tools then Follow Me tool. Now click on the curved surface and you'll get a comlete curved edge around the entire cylinder.

Now you want to select the curved surface and the panel it's sitting on. Right click and then click on Make Group. With the New Group still selected Right click again and then click on Soften/Smooth Edges. A new window will open up and then you can click and drag on the pointer and adjust the smoothness.

Now all that's left is the stop Top and Bottom circle surfaces left from the orginal cylinder, you can choose to keep or delete them like I choose to do.

I hope this helps you with your sketchup model. If you have any questions or if I misunderstood what you were asking don't be afraid to let me know.