Separate the list of station numbers (and/or names) with commas, and optional space. (Of course, that means you cannot use a comma in a station name.)
If you wish to adjust the weight accorded a station's sea-level record, you may prefix it with a number and an asterisk. E.g., Brest is PSMSL gauge #1 and Honolulu is #155, so “3*1,155” would give 75% weight to Brest and 25% weight to Honolulu.
Here are five more examples (which are all equivalent):
1*Brest, 1.5*Swinoujscie, 2.5*Honolulu Brest, 1.5*Swinouj, 2.5*Honol 2*Brest, 3*Swinoujscie, 5*Honolulu 2*1, 3*2, 5*155 2*190-091, 3*110-092, 5*1612340
Note that no month of data will be included in the graph and analysis unless it is present in all of the averaged sea-level records. That means you might want to omit sparse measurement records (like PSMSL #3, Sheerness) from the sites that you average.