IMDb will not accept credits at the Series level; they must be added to the individual Episodes.
However, if you were credited the same way in multiple Episodes, there is a way to enter them all in a single submission.
Start at the Series page, and click the Edit page button near the bottom, and proceed to fill out your credit (Select Add 1 credit beside visual effects; click continue; enter your name in the correct Surname, Givenname format and select visual effects supervisor). Note: if your name did not actually appear in the on-screen credits you must also enter (uncredited) in the Attribute field.
Once you have selected all the correct Episodes click the Close button on (Re-)Check these updates.
The rest is straightforward, and you should be able to submit your credits.