I think you're saying you want a multi value field in order to store more than one value in a field in a particular record, as well as have a validation rule - two issues.
Many experienced Access developers will tell you to avoid mvf's like the plague. They violate basic normalization rules, and frankly it amazes me that MS saw fit to create them in the first place, but to do so is your privilege.
As for the validation, I've no idea where you want to do this. In code? If so, use the form control AfterUpdate event.
In the validation rule property? ="M" Or "T" Or "W" (or whatever values you choose) and write a validation message in the Validation Text property.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.