To add parameters to Explore, click on Account Settings>Parameter Names. To find a particular parameter, simply type the name in the search box in the upper right, then click on Add to Explore in the Display Name column. You will be asked to provide a Display Name, and a Category which will be used to when selecting the parameter in the view.
This will start indexing of the parameter. The display Name column will show the word Indexing while this process is ongoing.
Note that parameters in explore are listed differently than they are in the log files or Analytics. Underscores "_" are used instead of periods as separators and the prefix "data" is added to the name. So WT.ac becomes data_wt_ac.