There is no problem for a constant outcome if you are only using MA(1) and C in the fomular and choosing Dynamic in the forecast, the constant you see in the forecast should be equal to the C in your function. This is because Move Average is assuming the terms are all white noise, so their expectation in the future are 0.
But, if you choose Static in the forecast, it will give you different result, it is using the Theta multiply t-1 residual to estimate t time residual. The detail explaination can be found here:
http://forums.eviews.com/viewtopic.php?f=7&t=465