The strumming pattern is a combination of 16th note and 8th note strums which can be a bit tricky.

The Em chord is:

1 8th note strum (downstroke)
2 16th note strums (down, up)
1 8th note strum (downstroke)
1 16th note strum (downstroke)

Then it switches to the D chord which, is all upstrokes:

1 8th note strum (upstroke)
1 8th note strum (upstroke)
1 16th note strum (upstroke)

Then to the Cadd9 chord:

1 8th note strum (downstroke)
2 16th note strums (down, up)


The tricky thing about this is that switching to the D chord occurs one 16th note before the 3rd beat of the bar, instead of right on that 3rd beat. So you are in full strum there by hitting the Em with a downstroke, then switching to the D and hitting it with the upstroke.

I really hope this helps you out ... let me know if there is anything else i can do.

Keep rocking!
Mike