When using an alternate index, you simply specify the name of the path in the DSNAME parameter of the DD statement associating the base data set/alternate index pair with your PL/I file. Before using an alternate index, you should be aware of the restrictions on processing; these are summarized in Table 26.
Given a PL/I file called PL1FILE and the alternate index path called PERSALPH, the DD statement required would be:
//PL1FILE DD DSNAME=PERSALPH,DISP=OLD