In the quick mission generator? Look at the picture of the sub in the left corner. Select the drop down box below it. Pick what platform you want.
In the normal mission editor place a sub on the map, select the project 971, on the second page for that item select the playable platform option.
For the random enemies place all the different ships or subs you want on the map and use dynamic groups on the platforms. Make sure they have a prob of inclusion equal to the number of ships in the group (IE: 2 = 50% each, 4 = 25% each and so on). Use each group to add a different platform on the map. You can also have groups in groups IIRC. Also use Random boxes so they will appear at different spots on the map.
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