What is the complement of a language?
Remember that a language is defined as a set of strings. The complement of a language is thus the complement of that set, defined in the usual way: everything not in the set.

In practice, when talking about the complement of a language, there's usually a particular alphabet you're interested in (which you can infer from context). If all else fails, assume $\{0,1\}$.

So in this case, the complement of that language is:


  The set of all binary strings $s$, such that either $s$ isn't a valid encoded Turing machine, or the machine encoded by $s$ accepts $1010$.


Hint: the problem of whether a string $s$ is a valid encoded Turing machine or not is known to be decidable. So you only need to worry about the second clause.