micromole/L is micromolar. Primers are often shipped in a lyophilized (dried/powdered) state. In many labs, after resuspension, these primer stocks are kept at 100 micromolar - hence, it is simply easier to write in the methods that a certain molar concentration was used (versus an amount in nano or picograms).
For instance, we often get primers shipped that are 30nanomoles. After adding 300ul of water, these primers will be 100 micromolar.
So, to answer your question, X micromole does not mean to use X microliter. It would depend on the concentration of your primer stocks.