It depends on the MIC appeared.You can use the MIC concentration or more in 0.5% DMSO and use the D MSO oly as a negative control and use known antibiotic to be positive control
To solve your problem, perhaps you can use the concentration of DMSO solution for your extract as your negative control. So that you can refer to it for the differences.
On the other hand, may be you can increase the solubility of your extract by using a sonicator when you dilute it in DMSO, try to grind the extract before hand, or try to use a slight heat to dissolve.