They haven't done it yet. Obviously a win over the Sparks tomorrow would put them in. However, if Seattle loses it would put the Lynx in even if they lost to LA. 

A Storm loss puts them at 13-13 while the Sparks would be at 11-14 with a win over the Lynx. Minnesota can't do worse than 19-15. Seattle and LA have two games remaining against one another, meaning they can't both finish at 20-14 or better. The Lynx own the tiebreak over both teams. 

"What about multiple team ties involving San Antonio and/or Phoenix?", I hear you asking.

Adding San Antonio to a tie with LA is no help at all. The SASS can do no better than 2-2 against Minnesota and no worse than 2-2 against the Sparks, which would leave Minnesota's winning record over LA as the deciding factor. For a three way with San Antonio and Seattle, the Storm would have to beat the SASS twice (because they'd have to lose both game to LA to keep them out of the tie) which would put Becky Hammon and company at 1-4 against the Storm and way out of the running. A four way tie involving the SASS with LA, Seattle, and Minnesota is obviously favorable for the Lynx, as they would have a .500 or better record head-to-head with all three teams. 

In a Phoenix-LA-Minnesota three way, the Merc lose their remaining game to the Sparks the head-to-head records would be LA 5-4, Minnesota 5-5, Phoenix 4-5 and the Merc would be out. If the Merc win their remaining game with LA, the Sparks would have to beat Seattle twice to get into the tie, which would either put the Storm out (if they lost another game) or force a four way tie.

In a Phoenix-Seattle-Minnesota three way, if the Merc lose their remaining game to the Storm the head-to-head records would be Minnesota 5-4, Seattle 4-4, Phoenix 4-5 and the Merc would be out. A Phoenix win over Seattle would mean the Storm have to split with LA to get into the tie, which would either put the Sparks out (if they lost another game) or force a four way tie. In a four way Phoenix-Seattle-LA-Minnesota tie, the Lynx would be 8-6 head-to-head with the others and would be in. 

In the event of a Minnesota-LA-Phoenix-San Antonio four-way, San Antonio would have to lose both games to Seattle (to keep them out of the tie) and one to LA (to keep them in). That would mean they could only lose one of two to Phoenix. If they beat the Merc twice the head-to-head records would be LA 7-6, San Antonio 7-6, Minnesota 7-7, and Phoenix 6-8. If they split, it would be LA 7-6, Minnesota 7-7, Phoenix 7-7, San Antonio 6-7. Either way the Lynx are in. 

For a Minnesota-Seattle-Phoenix-San Antonio four-way to matter, the Sparks would have to win out. That would mean Seattle has to win all their non-LA games to stay in the tie. That adds three losses to San Antonio, which means they have to win at least one of their two remaining games with Phoenix. If they split, the head-to-heads would be Seattle 8-5, Minnesota 7-6, Phoenix 7-7, San Antonio 5-9. If the SASS win both, it would be Seattle 8-5, Minnesota 7-6, Phoenix 6-8, San Antonio 6-8. Either way, the Lynx are in. 

There would still be the possibility of a five-way. There are several possible breakdowns, but in all of them the Lynx are 10-8 head-to-head and in the playoffs. I could run through them all, but suffice to say that the five-way is not what you want to see if you are a Merc fan.