I don't think wildcards can be used here.
Maybe this approach works for you (first commands just copied from above):
dt.rawdata = Open( Pick File());
// Create string that is filename
dtname = dt.rawdata << Get Name;
// Create variable called siteid that removes just the site name from the filename
If( Word( 1, dtname, "_" ) == "UC",
siteid = "UC_" || Word( 2, dtname, "_" ),
siteid = Word( 1, dtname, "_" )
);
// Create list of files in directory
path = "$dir_metplots/";
files = Files In Directory( path );
n = N Items( files );
// Get name of the file that starts with current siteid
For( i = 1, i <= n, i++,
If( Starts With( files[i], siteid ),
filename = files[i];
Break()
)
);
// Open file
dt.metplots = Open( path || filename );